You can get it from here.
Apt-rpm - http://apt-rpm.tuxfamily.org/
Once installed, as root type:
apt-get install xmms-mp3
This will download and install the codec for mp3.
If you don't have XMMS installed type this.
apt-get install xmms
You may want to also type apt-get dist-upgrade
this will check your RPM database and upgrade any programs that have newer versions available.