@williamsjohn. Thanks for posting. I tried out your like and I was able to upload an mp3 file. Here’s my screenshot: http://imageshack.us/photo/my-images/577/mp3success00.png/
If you are getting a particular error, you can make a screenshot and post the link here. That way, I can see the problem, and advise you better.
The mime type that was detected for the mp3 that I uploaded is “audio/mpeg”, as you can see from my screenshot.