Go here, do as it says: http://rpmfusion.org/Configuration
After that, depending on what players you use, just do:
yum install gstreamer* xine-lib-extras-freeworld smplayer vlc
gstreamer has a ton of plugins, I don't remember all of them. It will also install it's devel, but who cares. Install it only if you use gstreamer framework based players, like Totem.
I'm puzzled though. You have ~850 posts here and don't know how to get mp3 playback? Are you kidding? If yes, than you got me. Congrats.