Fixed popping noise upon stopping a sound by quickly ramping down the volume instead of stopping immediately. Also did a bit of additional housekeeping.

engine/Quake/bgmusic.c

@@ -132,6 +132,12 @@ qboolean BGM_Init (void)
 void BGM_Shutdown (void)
 {
 	BGM_Stop();
+
+	if (bgm_channelGroup)
+	{
+		FMOD_ChannelGroup_Release(bgm_channelGroup);
+		bgm_channelGroup = NULL;
+	}
 }
 
 static qboolean BGM_PlayStream(const char *filename)

engine/Quake/snd_fmod.c

@@ -148,6 +148,12 @@ void S_Shutdown(void)
 		}
 	}
 
+	if (sfx_channelGroup)
+	{
+		FMOD_ChannelGroup_Release(sfx_channelGroup);
+		sfx_channelGroup = NULL;
+	}
+
 	// If we created the FMOD System (and consequently own it), destroy it here
 	if (fmod_ownership)
 	{
@@ -221,9 +227,6 @@ static FMOD_RESULT SND_FMOD_Callback(FMOD_CHANNELCONTROL *channelcontrol, FMOD_C
 		return FMOD_OK;
 	}
 
-	// Clear the channel to signify that it's free
-	userdata->channel = NULL;
-
 	if (userdata->zone)
 	{
 		Z_Free(userdata);
@@ -255,6 +258,7 @@ static void SND_FMOD_SetChannelAttributes(FMOD_CHANNEL *channel, sfx_t *sfx, vec
 static soundslot_t *SND_PickSoundSlot(int entnum, int entchannel)
 {
 	soundslot_t *slot;
+	unsigned long long dspclock;
 
 	if (entnum < 0 || entnum >= MAX_CHANNELS || entchannel == 0 || entchannel > 7)
 	{
@@ -272,7 +276,10 @@ static soundslot_t *SND_PickSoundSlot(int entnum, int entchannel)
 	if (slot->channel)
 	{
 		// Stop any sound already playing on this slot
-		FMOD_Channel_Stop(slot->channel);
+		FMOD_ChannelGroup_GetDSPClock(sfx_channelGroup, &dspclock, NULL);
+		FMOD_Channel_SetFadePointRamp(slot->channel, dspclock + 64, 0.0f);
+		FMOD_Channel_SetMode(slot->channel, FMOD_LOOP_OFF);
+		slot->channel = NULL;
 	}
 
 	slot->zone = false;
@@ -489,6 +496,7 @@ void S_StaticSound(sfx_t *sfx, vec3_t origin, float vol, float attenuation)	// N
 void S_StopSound(int entnum, int entchannel)
 {
 	soundslot_t *slot;
+	unsigned long long dspclock;
 
 	if (!fmod_system)
 		return;
@@ -499,7 +507,12 @@ void S_StopSound(int entnum, int entchannel)
 	slot = &entsounds[entnum].slots[entchannel];
 	if (slot->channel)
 	{
-		FMOD_Channel_Stop(slot->channel);
+		// Instead of immediately stopping the sound, we fade it down to 0 volume over several samples and let it play out
+		// This prevents an annoying popping noise, which is especially noticeable on rapid-fire weapons
+		FMOD_ChannelGroup_GetDSPClock(sfx_channelGroup, &dspclock, NULL);
+		FMOD_Channel_SetFadePointRamp(slot->channel, dspclock + 64, 0.0f);
+		FMOD_Channel_SetMode(slot->channel, FMOD_LOOP_OFF);
+		slot->channel = NULL;
 	}
 }