Also make sure that your "mp3s" aren't AAC encoded. Examples *.m4p (Stuff you buy from the iTunes store) *m4a (Default ripping and encoding on iTunes). Those won't work on the GP2X.
I'm attempting to port the jfbuild engine but I have come across my first obstacle. It utilizes the fmod engine wich is a closed source library (unless you're willing to pay a hefty commercial fee) with no arm binaries.
I will have to rewrite sound.c <_< What would be a good alternative...