Where are you getting the .mp3 from?

The most important thing to remember when uploading music, is that if you take the file directly from iTunes, more often than not, the song will be .aac NOT .mp3.

If you are using Itunes go

Preferences > General > Import Settings.

Then select "import using mp3 encoder" and select the "good quality" (128kbps - this is the optimum bit rate for the UG player). Then try uploading it. The conversion can also be made using audacity, im just not too sure how, and if its still online, www.zamzar.com should be able to convert for you too.