How difficult is it anyway? Looks to be the proverbial final frontier of rom hacks. Nigh everything from music, voices (undubs), to even FMV (subtitle routines) is brought up on occasion.

Yet sound effects... Few utter mere suggestion of the concept. I don't blame people nonetheless. Too few games have shown a real need for SFX improvements, the vast majority seem to be viewed as adequate enough.

It's not always as simple as changing the BRR, in the case of SNES. Often there is some kind of manipulation of the sample as it is being played continuously - pitch slide, vibrato, or some other kind of modulation that would take up too much space in memory if it were actually recorded that way. What all happens is really up to the SPC engine the game uses. If the original sample is being messed with during playback, any new sample you insert will also have the same things applied to it. It's really hard to speak in general b/c the combo of the SPC and 65c816 can be fairly flexible.