Because of licensing reasons there are is no codex for the mp3 format included in Fedora by default. If you want to play mp3 files you need the following additional packages:
gstreamer-plugins-ugly gstreamer-plugins-bad gstreamer-ffmpeg
This packages are available in the RPMForge repository. Install this two RPMForge repositories:
rpm -Uvh http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-stable.noarch.rpm
rpm -Uvh http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-stable.noarch.rpm
Now you should be able to install the needed packages using yum:
yum install gstreamer-plugins-ugly gstreamer-plugins-bad gstreamer-ffmpeg
Now you should be able to play mp3.