Make sure you've got synaptic installed (as opposed to kynaptic and other poor clones of it). Then, under the Settings menu you should find a Repositories entry. Go there, and Check all of the Binary checkboxes. Then exit from the menu and click on the Reload icon.

The available packages should now be greatly expanded from before, and there will be lots of .mp3 and dvd codecs and the like. Use the Search tool to find mp3 in Name and Description for starters.

-ml