From Amarok Wiki
First, to enable MP3 support, the rpmfusion repository should be added to Yum. Configuration instructions for the rpmfusion repository can be found here.
Once rpmfusion is enabled, the easiest way to enable all the freeworld/nonfree codec support, is to:
yum update @sound-and-video
However, this may pull unwanted or unneeded packages. If so, you can install just the mp3 codecs as follows.
For Amarok 2 using phonon-backend-xine:
yum install xine-lib-extras-freeworld
For Amarok 2 using phonon-backend-gstreamer:
yum install gstreamer-plugins-ugly
More info can be found at FedoraFAQ