1st go to this page and download the mp3 plug-in :
http://home.gagme.com/greg/linux/fc3-tips.php
-> install it (there is instruction there.... )
-> go to xmms prefernce ->audio->unchecked/disable the mp3 place holder .......
-> the try to restart your pc ( the plug-in u just install did not appear - in my pc also)
-> try look again whether it appear or not(the new plug-in), it should be under the placeholder plug-in...
-> if not try dload this plug-in ( try search) xmms-mpg123-1.2.7-21.i386.rpm.. i get this from link given in this forum but i cannot find it right now .. the topic still under xmms...sorry
-> try to install it ... at this stage when i try to install a message appear telling me that there is another plug-in xmms-mp3-1.2.10-11.1.1.fc3.rf.i386 (that u insatll earlier)
-> try open again the xmms .. look at the audio option whether there is MPEG LAYER 1/2/3 Player (libmpg 123.so)... if it appear , enable it......then try play mp3 song.....if the message appear telling that ur sound card not blok bla bla bla .....
try to set the output plug-in.. for me some times it require eSound but sometimes it require ALSA .....
-> try it .. i didńt try for 2nd time with the same step like above but i hope it works.... maybe il try sooner... i hope it works... u can private msg me ... maybe we can exchange xperience using fedora

.. if i can help u .. i will help byee...