hmmm... what distro are you using??? what music player???
(i'm guessing red hat 9 with xmms...???)
some distros (such as red hat) don't include mp3 support out-of-the-box because of legal reasons (mp3 compresion is NOT free software)...
so what you do is you download the mp3 support for your xmms after the install...
you can get the mp3 support for xmms for red hat 9 here: