Try all of these possible fixes in this order:
1.) Make sure what you're submitting is an MP3 file.
2.) Make sure it's sampled at 44.1 KHz (you can check this in Audacity).
3.) Make sure it's less than 15 MB.
4.) Clear your cache, refresh the page and try again.
5.) Change the filename. From my experience, filenames which are too long or have special characters such as '#' in them may fail to upload.
6.) Try a different browser.
7.) Re-export the MP3 file and try again, taking all the above precautions.
8.) Send a clearly-named bug report via PM to liljim outlining your problem with details, links and screenshots.