This is completely a wrong forum (website suggestions & feedback). But..
You'll probably need a plugin for your mp3 files. If you've installed the default Ubuntu installation, you'll need either gstreamer plugins or xine plugins; which app do you use to play mp3? Do you use Totem? If so, open Synaptic and see if you've got "Totem-xine" or "Totem-gstreamer" installed, and download then libxine-extraplugins or gstreamer's plugins accordingly.
If you're unsure you can first try downloading xine-plugins since that's what I guess you use; in console, do
sudo apt-get install libxine-extracodecs
and retry playing an mp3 file. If that doesn't help, open Synaptic and get gstreamer-plugins.