Not a straight-forward task, senor
I followed the instructions here:
http://www.holovaty.com/linux/fedorafaq/kde_mp3.html
Don't try that if you are not sure about what you are doing
Alternatively, you can check out the kde-redhat project here: http://kde-redhat.sf.net
They might have mp3 support in their kde packages, but I'm not sure.
--R