I got XMMS running MPEG-4 Audio files the plugin on the site was pretty ugly and I couldn't make heads or tails of it. but there was an rpm that seemed to work here:
that got my files running. though there was some problem with XMMS freezing up on some very large files (20+ megabytes) I think it had something to do with the compression, as they were very large in filesize for the leingth of the song.
well good luck.