Classic for the lowest latency, smooth for less glitches when running at speeds other than 100%.
Turns on or off sound globally.
Sets the overall volume of sound during gameplay.
Lets you separately control the volume of reverb and echo effects. PPSSPP's implementation of these isn't 100% accurate and it can be a little loud in some cases, or maybe you just don't like reverb - if so, just turn this down a bit.
Audio isn't very clean when running at alternate speeds, especially when Playback mode is set to Classic, so if you don't like that, turn this down.
This controls built-in sound effects when navigating the UI.
When selecting games on the main screen, some will play the built-in music loop supplied on the disc. These might be too loud, so you can turn them down here.
Choose your audio device here, or leave it at Auto.
If this is checked, sound will move to any new audio devices that you plug in.
Here you can pick the device to use for PSP microphone emulation. Very few games make use of this.