That's because right now NG's audio uploading tool is really glitchy right now. It has always been fairly buggy, but since the redesign it has become a massive pain to try and upload MP3 files, especially large ones. The only fix right now is to keep on trying - try again and again, and if that doesn't work then try a different browser.
So far, other than doing repeated attempts, I haven't heard of any way to get around it. The staff know about the problem and are working on fixing it once that get more pressing matters out of their hair.
Good luck uploading that file!