Is it possible to replace the mp3 files in an iso+mp3 setup, to use whatever music you'd like? And if so, are there any certain guidelines in doing it, such as bitrate, or making sure the track length matches the mp3 you're replacing?

Could we have more detail on the situation?

Well, my version of Guardian Heroes is an iso, with all the tracks in Mp3 format (which I'd have to convert to wav format anyways), and I wanted to change some of the music tracks to some other music I have.

I suppose you could do it simply by matching the filenames...

I did a little test with Lords of Thunder on Sega CD, and it ended up crashing Kega Fusion So maybe I'll have better luck with cassini.