Mp3 support was removed from Fedora due to licensing issues. If (when) you want to add it back, you can check out http://rpm.livna.org for a fairly simple method. Just go down to where it says "For Fedora" and add the specified lines to /etc/yum.conf (ignore the stuff about apt and up2date unless you're already using them). Once your /etc/yum.conf is updated, you can simply type yum install xmms-mp3 at the console (as root), and yum will do the rest. When it's finished installing, just run xmms and you should be on your way.