Alternative speed and Sound speed

[LunaMoo]

You can't. PPSSPP is an emulator, not a porting tool which would allow customizing how games run. We don't just run some sound files on top of a game, but emulate the game which decode and plays the sounds it wants itself, so asynchronized sound is impossible without lots of problems.

That being said you can affect game speed with cheats/hacks without affecting sound. Lots of 60fps patches have to do exactly that, but the other way around, that is they unlock more fps, then slow the game down. Commonly controlling game speed is harder than just unlocking fps through, but doing just the latter most of the time ends up running games faster with correct sounds, so could be good enough for your need. You can check this discussion for more info.