There are lots of programs to play .mp3 files. Amarok, Audacious, VLC, Juk and Xmms just to name a few. Of course you need a working sound system. There are decoders for .mp3 files that some distros do not install by default.
Don't know about RHEL 5, never used it. Look under Multi Media in your menu, if there is a player installed, it should be found there.