I read this on ximian web site regarding xmms:
Due to licensing restrictions on MP3 playback and decoding, and legal ambiguity surrounding the role of Free software with respect to those licensing restrictions, Ximian is unable to ship a free MP3 player.
If you wish to play MP3 files you have a number of options:
The paid version of Ximian Desktop 2 includes RealPlayer, which can handle MP3 files as well as RealAudio files.
The website xmms.org has information about MP3 patent issues and suggestions for finding MP3-enabled players.
The OGG file format is unencumbered by patent restrictions and is playable by XMMS and many other players. You can learn about it at Xiph.org.