By default, the codecs to play mp3/mp4 are not included due to licensing issues. First you need to add the non-standard repositories:
rpm -ivh http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-stable.noarch.rpm
rpm -ivh http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-stable.noarch.rpm
rpm --import /etc/pki/rpm-gpg/RPM-GPG-KEY-rpmfusion-*
That done, if you just double click on a mp3 file (starting Fedora 8 onwards), it will pop-up a dialogue saying that you must install missing codecs. Click on yes and go along.
You can also explicitly install them like this:
yum install gstreamer-plugins-ugly gstreamer-plugins-bad gstreamer-ffmpeg
Further reading: