Fedora 9 does not release native support for MP3, so you need to get the right bits from another source like rpm.livna.org. First, install Livna's release package for Fedora 9. This will provide you with access to Livna.org's Fedora 9 YUM repository using /etc/yum.repos.d/livna.repo.
rpm -ivh http://rpm.livna.org/livna-release-9.rpm
Now, for XMMS MP3 support, install xmms and xmms-mp3
yum install xmms xmms-mp3
For Amarok MP3 support, install amarok and amarok-extras-nonfree.
yum install amarok amarok-extras-nonfree
For xine with MP3 support, install xine and xine-lib-extras-nonfree
yum install xine xine-lib-extras-nonfree
To create MP3s with LAME, install lame and lame-mp3x
yum install lame lame-mp3x