You have to add the repository to be able to get plugins for mp3

yum localinstall --nogpgcheck http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-stable.noarch.rpm http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-stable.noarch.rpm

Then ad the actual plugin

yum install gstreamer-plugins-ugly gstreamer-plugins-bad-nonfree