2 is the magic number
"On any given Sound Cue, volume settings up to around 2.0 will increase the perceived loudness of the audio file, and anything above that would not. A single cue will never distort, but you would not want all of your files at maximum volume because it will likely overload when multiple cues play simultaneously in-game" So there you have it folks, don't pass that threshold or you'll end up with tubes from the rain forest.