Audio Settings
Enable sound
Turns on or off sound globally.
Global volume
Sets the overall volume
Alternate speed volume
Audio isn't very clean when running at alternate speeds, so if you don't like that, turn this down.
Reverb volume
Lets you separately control the volume of reverb and echo effects. PPSSPP's implementation of these isn't 100% accurate yet and can be a little loud in some cases.
Audio backend
If you're on Windows and sound is strangely choppy even though games are running at full speed, try changing this to DSound. This setting is not available on all platforms.