playing mp3 files in linux

First of all why install Red Hat 9? It's a very old distro. There are mant newer and better distros now like Fedora Core 5, Ubuntu 6.06, Suse 10.1, Mandriva 2006 and many others to choose from.
Decide which one you want then refer to the sticky post in this section on how to enable multimedia in different distros, your question will be answered.

check the goodies section in gurulabs.com for the xmms-mp3 plugin for RH9