Grrr wiped out my post because I didn't have enough posts to post a URL. Not sure where to get points so just google the file. I used the DAG version of it. If you have DAG set up in your YUM you should be able to yum it also. I would post my dag yum config but it also contains a URL and would make me retype the whole thing again
k3b-mp3-0.11.14-1.2.el4.rf is the file I think your looking for. This will allow you to convert MP3s to audio files in K3b. You can do the same manually before burning or convert them to waves but the sound quality is good enough with this plugin and it's so much easier that I use the plugin %99 of the time. The RPM is for FC4 but installed fine in FC3, though Yum complains when I try to update K3b because of this. Don't see an FC5 rpm either. I run 3 and 5, hated 4. Ran it only long enough to decide I didn't want too.
Google that RPM and you can then burn MP3s as audio fails inside K3b.