MP3 support is not included in Redhat or Fedora due to licensing concerns. You will need to first install MP3 support for XMMS in order to play files in that format.
The necessary RPM file (xmms-mp3) is available from Freshrpms.net .
Here's what I did on FC3 to download and install the plugin:
$ wget http://ftp.freshrpms.net/pub/freshrpms/fedora/linux/3/xmms/xmms-mp3-1.2.10-9.1.1.fc3.fr.i386.rpm # rpm -ivh xmms-mp3-1.2.10-9.1.1.fc3.fr.i386.rpm
While you're at it, you may also want to install the skins rpm and check out some additional XMMS tips at the LinuxDevCenter .