1. Overclock your Pi – You MUST use heatsinks/fan for this
2. I haven’t tried this yet but it is on the wiki for this project: in your retroarch.cfg file, set the audio_driver to sdl (or try alsa, see which is best) and set the audio_out_rate to 44100
3. Try the “experimental” ARMSNES emulator, it has better sound but less compatibility with games…

let us know what works best for you :)