Hi Simon,
I was just drawing reference to the fact that DrJeep gave you a way to find this out already (by installing mpg321).
You know, it almost seems like you didn't read my response, and never tried what DrJeep suggested at all.
The idea is this: go back and read said email, try what it offers, and that will probably answer the question you have asked here.
Once you have taken that first step, if all is well, google for "playing mp3 on ubuntu", choose what I suspect is still the first result regarding RestrictedFormats and ascertain what is appropriate/OK for you to do from that page.
Needless to say, if you read the suggestions presented to you in this email, the chances of them helping you will drastically (hopefully!) improve.
Good luck,
Marcus