Howtogeek posted a guide about enabling MP3 support in Ubuntu 10.04

Because of licensing issues, Ubuntu is unable to play MP3s out of the box. We?ll show you how to play MP3s and other restricted file formats in about four mouse clicks.
The philosophy behind Ubuntu is that software should be free and accessible to all. Whether MP3 and other file formats are free is unclear in many countries, so Ubuntu does not include software to read these file formats by default.

Fortunately, it does include a package that installs the most commonly used file formats all at once, including a Flash plugin for Firefox.
Note: These instructions are for Ubuntu 10.04. There are small differences for earlier versions of Ubuntu.

  How to Make Ubuntu Play MP3 Files