Set channels with 0 volume to automatically become virtual. This greatly reduces the number of real channels that are being used.

Assets/Scripts/AudioManager.cs

@@ -83,7 +83,7 @@ public class AudioManager : MonoBehaviour
         result = fmodSystem.setSoftwareChannels(MaxRealChannels);
         CheckFmodResult(result, "FMOD.System.setSoftwareChannels");
 
-        result = fmodSystem.init(MaxVirtualChannels, FMOD.INITFLAGS.NORMAL, IntPtr.Zero);
+        result = fmodSystem.init(MaxVirtualChannels, FMOD.INITFLAGS.VOL0_BECOMES_VIRTUAL, IntPtr.Zero);
         CheckFmodResult(result, "FMOD.System.init");
     }
 

engine/Quake/snd_fmod.c

@@ -59,7 +59,7 @@ void S_Startup(void)
 			return;
 		}
 
-		result = FMOD_System_Init(fmod_system, MAX_CHANNELS, FMOD_INIT_NORMAL, NULL);
+		result = FMOD_System_Init(fmod_system, MAX_CHANNELS, FMOD_INIT_VOL0_BECOMES_VIRTUAL, NULL);
 		if (result != FMOD_OK)
 		{
 			Con_Printf("Failed to initialize FMOD System: %s\n", FMOD_ErrorString(result));