This post explains how to install MP3 and other restricted codecs in Fedora 13, Goddard.
Issue these below commands from the terminal.
1. rpm -ivh http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-stable.noarch.rpm
2. rpm -ivh http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-stable.noarch.rpm
3. yum -y install gstreamer-plugins-bad gstreamer-plugins-ugly xine-lib-extras-nonfree gstreamer-ffmpeg
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Take care,
Ananth S Gouri