Well, I never tried to mess around with sound as I have no much info about it.
As far as I know, there is a folder named Sound, in which many, many files are stored with DSP format. There are one for the Left channel and another for the Right. There's also Samp and Info files. I found a Winamp Plugin that gives me the option to listen to the music and sound effects in these files.
I can tell you that most of the music is missing just by looking at the DSP files. Most of them are sound effects of different types.
I found four themes with that format, but they are all "the special music" like the intro theme or ending theme. And those sounds nicer than the usual music, which brings me to this (probably so logical that makes me look stupid) idea:
I think that most music is some sort of midi-esque affair. You have an info file that may store the information of each theme's composition, and a sample file with the instruments for each theme.
It would make sense as I remember the Maramba theme song having some of the instruments cut when you walk (Like if the cnannel for that instrument would be used for the footsteps sound effect or something). If the game had entire tracks that wouldn't be happening. I don't know how the original game did this (if they were sound tracks instead of these, I guess, sequenced formats).
All I know is that it's sadly beyond my abilities and knowledge. Even then, de GC version doesn't have much free space, and they altered the music for that reason, fitting both discs of the original release into one.
Maybe someone could try to improve the quality of the "midi" instruments or something..., or maybe trying to change these info+samp files for DSP files if the size problem could be solved (or if they had their quality lowered a little).
Sorry of not having a better response...