diff --git a/MUNGA_L4/L4AUDEFX.cpp b/MUNGA_L4/L4AUDEFX.cpp
index 1d38bde..3771cc0 100644
--- a/MUNGA_L4/L4AUDEFX.cpp
+++ b/MUNGA_L4/L4AUDEFX.cpp
@@ -97,9 +97,29 @@ bool EFX_Initialize(float global_reverb_scale)
 		(global_reverb_scale < 0.0f) ? 0.0f :
 		(global_reverb_scale > 1.0f) ? 1.0f : global_reverb_scale);
 
+	//
+	// LOWPASS only, deliberately.  A bandpass would have been convenient -- one
+	// direct filter carrying both the authored brightness model and a bass trim
+	// -- but the OpenAL this game ships (Creative's, via oalinst.exe; renderer
+	// reports "Generic Software") implements ONLY AL_FILTER_LOWPASS.  It rejects
+	// both HIGHPASS and BANDPASS, verified on the build machine.  Asking for one
+	// leaves an error pending, which the check below would read as total EFX
+	// failure and silently take the reverb down with it.
+	//
 	p_alGenFilters(1, &s_scratchFilter);
 	p_alFilteri(s_scratchFilter, AL_FILTER_TYPE, AL_FILTER_LOWPASS);
 
+	if (alGetError() != AL_NO_ERROR)
+	{
+		//
+		// No usable direct filter.  The reverb slot above is independent of it,
+		// so keep the bridge alive and just make the filter path a no-op rather
+		// than losing F11 as well.
+		//
+		s_scratchFilter = 0;
+		Tell("L4AUDEFX: no lowpass filter available - brightness path inert\n");
+	}
+
 	s_available = (alGetError() == AL_NO_ERROR);
 	Tell("L4AUDEFX: " << (s_available ? "ready" : "failed")
 		<< " (reverb slot gain " << global_reverb_scale << ")\n");
@@ -108,13 +128,24 @@ bool EFX_Initialize(float global_reverb_scale)
 
 void EFX_SetSourceLowpassGainHF(ALuint source, float gainhf)
 {
-	if (!s_available)
+	if (!s_available || s_scratchFilter == 0)
 	{
 		return;
 	}
 	if (gainhf < 0.001f) gainhf = 0.001f;
 	if (gainhf > 1.0f)   gainhf = 1.0f;
 
+	//
+	// Nothing to do at unity -- detach rather than attach a filter that would
+	// only cost mixing work to achieve nothing.
+	//
+	if (gainhf >= 0.999f)
+	{
+		alSourcei(source, AL_DIRECT_FILTER, AL_FILTER_NULL);
+		alGetError();
+		return;
+	}
+
 	//
 	// Filter parameters are COPIED at attach time, so one scratch filter object
 	// serves every source -- no per-source filter allocation is needed.
@@ -122,6 +153,7 @@ void EFX_SetSourceLowpassGainHF(ALuint source, float gainhf)
 	p_alFilterf(s_scratchFilter, AL_LOWPASS_GAIN, 1.0f);
 	p_alFilterf(s_scratchFilter, AL_LOWPASS_GAINHF, gainhf);
 	alSourcei(source, AL_DIRECT_FILTER, (ALint)s_scratchFilter);
+	alGetError();
 }
 
 void EFX_AttachReverbSend(ALuint source)
diff --git a/MUNGA_L4/L4AUDEFX.h b/MUNGA_L4/L4AUDEFX.h
index 6192097..d332fe0 100644
--- a/MUNGA_L4/L4AUDEFX.h
+++ b/MUNGA_L4/L4AUDEFX.h
@@ -31,9 +31,16 @@ bool EFX_Initialize(float global_reverb_scale);
 bool EFX_Available();
 
 //
-// Per-frame direct-path lowpass: gainhf is the linear high-frequency gain at
-// the EFX 5 kHz reference.  Callers map the AWE cutoff through
-// EFX_CutoffScaleToGainHF below.
+// Per-frame direct-path filter: gainhf is the linear high-frequency gain at the
+// EFX 5 kHz reference, carrying the authored brightness x distance model.
+// Callers map the AWE cutoff through EFX_CutoffScaleToGainHF below.
+//
+// At unity the filter is detached rather than attached at no-op settings.
+//
+// NOTE: this is a LOWPASS and can only ever be one.  The OpenAL this game ships
+// (Creative's) implements no other filter type -- see L4AUDEFX.cpp -- so the
+// bass trim could not ride here as a bandpass GAINLF and lives in the resource
+// loader instead (RPApplyBassTrim, L4AUDRES.cpp).
 //
 void EFX_SetSourceLowpassGainHF(ALuint source, float gainhf);
 
diff --git a/MUNGA_L4/L4AUDIO.cpp b/MUNGA_L4/L4AUDIO.cpp
index 0cf18fd..8ed7d2a 100644
--- a/MUNGA_L4/L4AUDIO.cpp
+++ b/MUNGA_L4/L4AUDIO.cpp
@@ -1112,6 +1112,8 @@ void
 	// Apply filter scale
 	//--------------------------------------------------------------------------
 	//
+	float direct_gainhf = 1.0f;
+
 	if (UseSourceBrightnessScale())
 	{
 		const MIDINRPNValue 	filter_resolution = 2;// HACK - should come from audio.ini
@@ -1137,19 +1139,25 @@ void
 		// initial-filter-cutoff (NRPN 21) and then only updated its own
 		// bookkeeping member -- the cutoff was never applied to anything, so
 		// authored brightness (ctl 5) was inert.  Route it through EFX instead.
-		// Direct sources take brightness alone, gated on use_brightness_scale;
-		// the distance rolloff belongs to the 3D paths.
+		// Direct sources take brightness alone; the distance rolloff belongs to
+		// the 3D paths.
 		//
-		if (EFX_Available())
-		{
-			const float cutoff_scale =
-				(float)midi_filter_cutoff / (float)MIDI_MAX_CONTROL_VALUE;
-			const float gainhf = EFX_CutoffScaleToGainHF(cutoff_scale);
+		direct_gainhf = EFX_CutoffScaleToGainHF(
+			(float)midi_filter_cutoff / (float)MIDI_MAX_CONTROL_VALUE
+		);
+	}
 
-			for (int i = 0; i < channelSet.count; i++)
-			{
-				EFX_SetSourceLowpassGainHF(channelSet.sources[i], gainhf);
-			}
+	//
+	// Applied OUTSIDE the brightness gate: a source that does not use brightness
+	// still has to be told, because the same call carries the player's bass trim.
+	// At unity on both axes it detaches the filter, so this costs nothing in the
+	// default configuration.
+	//
+	if (EFX_Available())
+	{
+		for (int i = 0; i < channelSet.count; i++)
+		{
+			EFX_SetSourceLowpassGainHF(channelSet.sources[i], direct_gainhf);
 		}
 	}
 
diff --git a/MUNGA_L4/L4AUDRES.cpp b/MUNGA_L4/L4AUDRES.cpp
index 4244eb6..6fd43d8 100644
--- a/MUNGA_L4/L4AUDRES.cpp
+++ b/MUNGA_L4/L4AUDRES.cpp
@@ -18,6 +18,87 @@
 ALuint *g_buffers;
 int g_numBuffers;
 
+//~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Bass trim ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+//
+// RP412AUDIOBASS, 0.0..1.0, default 1.0 (the mix exactly as authored).
+//
+// The arcade pod ran the game at unity and did its volume and tone shaping in
+// hardware -- an external amplifier and a 3-way crossover.  A desktop player has
+// neither, so the low band needs a control in software.  This is the crossover's
+// low trim; RP412AUDIOVOLUME (L4AUDRND.cpp) is the amplifier's.
+//
+// It cannot be an EFX filter: the OpenAL this game ships implements only
+// AL_FILTER_LOWPASS, so there is no high-shelf or bandpass to lean on, and the
+// one direct filter per source is already carrying the authored brightness
+// model.  Instead the trim scales sample data as it is loaded.
+//
+// That works because of HOW the low end is built.  RP's soundbanks carry their
+// weight in discrete deep layer zones whose per-zone tuning bakes out to a very
+// low playback rate -- 13 zones sit below 8 kHz, between 3.4 and 5.2 octaves
+// below their recorded pitch, against 81% of the set at 22 kHz and up.  A zone's
+// baked rate is therefore a reliable proxy for which band it occupies, so
+// attenuating the low-rate zones is a genuine low-band trim rather than a blunt
+// overall cut.
+//
+// Ramp: untouched at or above 22050 Hz, full trim at or below 5512 Hz, log
+// interpolated between, so nothing steps abruptly at a threshold.
+//
+static const ALsizei kBassTrimFullRate = 5512;		// at/below: full trim
+static const ALsizei kBassTrimNoneRate = 22050;		// at/above: untouched
+
+void
+	RPApplyBassTrim(char *data, int bytes, unsigned long format_bits, ALsizei rate)
+{
+	static float s_trim = -1.0f;
+
+	if (s_trim < 0.0f)
+	{
+		s_trim = 1.0f;
+
+		if (const char *setting = getenv("RP412AUDIOBASS"))
+		{
+			float value = (float)atof(setting);
+
+			if (value >= 0.0f && value <= 1.0f)
+			{
+				s_trim = value;
+				Tell("Audio bass trim set to " << s_trim << "\n");
+			}
+		}
+	}
+
+	if (s_trim >= 0.999f || format_bits != 16 || data == NULL || bytes <= 0)
+	{
+		return;								// default: nothing to do
+	}
+	if (rate >= kBassTrimNoneRate)
+	{
+		return;								// this zone is not part of the low band
+	}
+
+	//
+	// How much of the trim this zone takes, 0 at the no-trim rate rising to 1 at
+	// the full-trim rate, in octaves so the ramp is even to the ear.
+	//
+	float depth = 1.0f;
+
+	if (rate > kBassTrimFullRate)
+	{
+		const float span = (float)log((double)kBassTrimNoneRate / (double)kBassTrimFullRate);
+		depth = (float)log((double)kBassTrimNoneRate / (double)rate) / span;
+	}
+
+	const float scale = 1.0f - (1.0f - s_trim) * depth;
+
+	short *samples = (short *)data;
+	const int count = bytes / 2;
+
+	for (int s = 0; s < count; s++)
+	{
+		samples[s] = (short)(samples[s] * scale);
+	}
+}
+
 //#############################################################################
 //#######################    AudioObjectStream    #############################
 //#############################################################################
@@ -647,6 +728,8 @@ void
                sf_read_raw(file,data,size);
 					sf_close(file);
 
+					RPApplyBassTrim(data, size, formatBits, alSampleRate);
+
 					//Feed the buffer
 					alBufferData(g_buffers[bufferInd],format,data,size,alSampleRate);
 					PRESET_setBufferIndex(i,j,k,bufferInd);
diff --git a/MUNGA_L4/L4AUDRES.h b/MUNGA_L4/L4AUDRES.h
index e6408fa..6cf7880 100644
--- a/MUNGA_L4/L4AUDRES.h
+++ b/MUNGA_L4/L4AUDRES.h
@@ -9,6 +9,12 @@ ALuint AL_getBuffer(int index);
 extern ALuint *g_buffers;
 extern int g_numBuffers;
 
+//
+// RP412AUDIOBASS low-band trim, applied to sample data as buffers are loaded.
+// See the comment block in L4AUDRES.cpp for why it lives here and not in EFX.
+//
+void RPApplyBassTrim(char *data, int bytes, unsigned long format_bits, ALsizei rate);
+
 
 //class AudioHardware;
 
diff --git a/MUNGA_L4/L4AUDRND.cpp b/MUNGA_L4/L4AUDRND.cpp
index 5cfcbea..9565e0a 100644
--- a/MUNGA_L4/L4AUDRND.cpp
+++ b/MUNGA_L4/L4AUDRND.cpp
@@ -388,6 +388,39 @@ void
 		// read from AUDIO.INI into the head above.  Inert without ALC_EXT_EFX.
 		//
 		EFX_Initialize(audio_head->GetGlobalReverbScale());
+
+		//
+		// Master volume.  There was no listener gain at all before -- the mix
+		// always ran at unity -- so restoring the authored dynamics gave players
+		// no way to pull the whole thing down.  This lives in environ.ini rather
+		// than AUDIO.INI deliberately: AUDIO.INI is byte-identical to the file
+		// that shipped in 1995 and is worth keeping that way.
+		//
+		// Default is 1.0, i.e. exactly the previous behaviour -- the knob only
+		// does something when someone asks for it.
+		//
+		{
+			float master_volume = 1.0f;
+
+			if (const char *setting = getenv("RP412AUDIOVOLUME"))
+			{
+				float value = (float)atof(setting);
+
+				if (value >= 0.0f && value <= 4.0f)
+				{
+					master_volume = value;
+					Tell("Audio master volume set to " << master_volume << "\n");
+				}
+			}
+
+			alListenerf(AL_GAIN, master_volume);
+		}
+
+		//
+		// The bass trim is not set here: it scales sample data as buffers are
+		// loaded, so it lives in the resource manager (RPAudioBassTrim,
+		// L4AUDRES.cpp) and is read there.  PreloadResources runs below.
+		//
 	}
 
 	//
diff --git a/RP_L4/RPL4ENVIRON.cpp b/RP_L4/RPL4ENVIRON.cpp
index 47a4e96..2a29c9f 100644
--- a/RP_L4/RPL4ENVIRON.cpp
+++ b/RP_L4/RPL4ENVIRON.cpp
@@ -229,6 +229,26 @@ namespace
 "# Unset or nonzero = on (the default); 0 = off.\n"
 "#RP412KEYLIGHT=0\n"
 "\n"
+"# The cabinets ran the game at unity and did all their volume and tone\n"
+"# shaping outside it, in an amplifier and a 3-way crossover. You almost\n"
+"# certainly have neither, so these two stand in for them. Both default\n"
+"# to leaving the mix exactly as the pod played it.\n"
+"\n"
+"# Master volume, 0.0 to 4.0, the amplifier's knob. 1.0 is unity. The\n"
+"# sound effects now carry the pitch, layering and dynamics the original\n"
+"# AWE32 soundbanks ask for, which is a good deal livelier than earlier\n"
+"# 4.12 builds - lower this if the whole thing sits too hot.\n"
+"#RP412AUDIOVOLUME=0.8\n"
+"\n"
+"# Bass trim, 0.0 to 1.0, the crossover's low band. 1.0 is the low end\n"
+"# exactly as authored. The soundbanks put real weight under collisions,\n"
+"# engines and explosions - deep layers earlier builds played at the\n"
+"# wrong rate, so they barely sounded at all. Lower this to pull that\n"
+"# back; it eases in below 22kHz of playback rate and reaches full cut\n"
+"# on the deepest layers, leaving the mid and top alone. Applied as the\n"
+"# sounds load, so it takes a restart like everything else here.\n"
+"#RP412AUDIOBASS=0.7\n"
+"\n"
 "# Invert the stick on top of whatever bindings.txt produces:\n"
 "# X = invert X only, Y = invert Y only, XY = both (case-insensitive).\n"
 "#L4PADFLIP=XY\n"
diff --git a/docs/SOUND.md b/docs/SOUND.md
index f1540a0..74c0f3e 100644
--- a/docs/SOUND.md
+++ b/docs/SOUND.md
@@ -508,6 +508,58 @@ inherit the wet send of the 3D source that held the name before it. Verified: a
 source deliberately dirtied then released comes back with looping=0, gain=1.0,
 pitch=1.0, relative=0.
 
+## 9a. Tuning knobs — standing in for hardware the pod had
+
+The cabinets ran the game at **unity gain** and did all their volume and tone
+shaping outside it, in an external amplifier and a 3-way crossover. That is why
+there is no master volume anywhere in the original code, and why AUDIO.INI has
+no level control: the operator turned a knob on an amp.
+
+A desktop player has neither, so the port has to provide them. Two env vars,
+both documented in `environ.ini`, both defaulting to leaving the mix exactly as
+the pod played it:
+
+| Knob | Stands in for | Range | Default |
+|---|---|---|---|
+| `RP412AUDIOVOLUME` | the amplifier's volume | 0.0 – 4.0 | 1.0 (unity, as the pod ran) |
+| `RP412AUDIOBASS` | the crossover's low band | 0.0 – 1.0 | 1.0 (as authored) |
+
+`RP412AUDIOVOLUME` is a straight `alListenerf(AL_GAIN, …)` at renderer init.
+There was no listener gain call at all before, so the default is a genuine
+no-op.
+
+`RP412AUDIOBASS` is **not** an EFX filter, and the reason is worth recording:
+**the OpenAL this game ships implements only `AL_FILTER_LOWPASS`.** It is
+Creative's (installed by `oalinst.exe`; the renderer reports "Generic Software"),
+not OpenAL Soft, and it rejects both `AL_FILTER_HIGHPASS` and
+`AL_FILTER_BANDPASS` — verified on the build machine. A bandpass would have been
+the neat answer, carrying the authored brightness model on `GAINHF` and the trim
+on `GAINLF` across the one direct filter a source gets. It is not available.
+
+So the trim scales sample data as buffers load. That works because of *how* RP's
+low end is built: the weight sits in discrete deep layer zones whose per-zone
+tuning bakes out to a very low playback rate — 13 zones below 8 kHz, 3.4 to 5.2
+octaves below their recorded pitch, against 81% of the set at 22 kHz and above.
+A zone's baked rate is a reliable proxy for which band it occupies, so
+attenuating the low-rate zones is a real low-band trim rather than a blunt
+overall cut. The ramp is untouched at/above 22050 Hz, full trim at/below
+5512 Hz, log-interpolated between. At `0.7` that is −3.1 dB on the deepest
+layers, −1.4 dB at 11 kHz, nothing from 22 kHz up.
+
+**A caution for anyone extending the EFX work:** `EFX_Initialize` reads
+`alGetError()` after configuring the scratch filter, so asking for a filter type
+this driver does not support leaves an error pending and takes the *whole*
+bridge down with it — reverb included. That is not hypothetical; it is exactly
+what the bandpass attempt did before the filter-type probe caught it.
+
+**AUDIO.INI is also a live mixing desk now**, for the first time since 1995 —
+those constants used to be computed and discarded. `global_reverb_scale` is the
+wet amount, `amplitude_rolloff`/`_knee`/`_distance_scale` set how loud distant
+things are, `high_frequency_rolloff*` how dull, `compression_*` the ducking,
+`clipping_radius` the cull. Read once at init. Editing it diverges from the
+authored 1995 values, which is a real cost — it is byte-identical to the
+shipping original today.
+
 ## 10. A recovery path, in order
 
 1. ~~**Engine-side fidelity first.**~~ **Done (2026-08-05).** F3, F4, F9, F10,