Ubuntu cannot include support for MP3 or DVD video playback or recording. MP3 formats are patented, and the patent holders have not provided the necessary licenses. Ubuntu also excludes other multimedia software due to patent, copyright, or license restrictions, such as Adobe Flash Player and RealNetworks RealPlayer.
That doesn’t mean you can’t play .mp3 files in Ubuntu, it just takes a bit of work (not much).
Follow these instructions to get mp3 and other multimedia support on your Ubuntu 12.04.
Medibuntu (Multimedia, Entertainment & Distractions In Ubuntu) is a repository of packages that cannot be included into the Ubuntu distribution for legal reasons (copyright, license, patent, etc) as explained above.