Yeah definitely try a different browser. Also clear your cache, I've had constant problems with Newgrounds that have to do with the cache and while it is a long shot, it might work. The problem may also lie in your song's file; make sure it's less than 15 MB, sampled at 44.1 KHz (default) and it's an MP3 file. Lastly, try uploading audio from the project system instead.
If all of that fails then send a bug report via PM to an admin (preferably liljim, PsychoGoldfish or TomFulp). Make sure you name it properly, add screenshots/links, and mention your browser/browser version and OS/OS version.
Hope I helped.