I'm having trouble with m4a files too. I have a whole folder of them in the MyDocs/music/Alison\ Moyet/Singles (for example).
When I use the file manager to view that folder, they're all there, and if I try to open them, Music Player opens and plays them just fine, but I don't see any of them if I launch Music Player and browse them.

Is there some method of getting them into music player?