Chris, as far as I'm aware, it has to be a URL that ends in ".mp3"
So, it would be "http://www.exampledomainname.com/audio/examplemusic.mp3" and that should work. So, instead of uploading it somewhere to save on bandwidth, you're going to have to upload it to your own server (or another service that will still allow the .mp3 in the URL) and use it that way.
I'm not 100% sure, but someone else may be able to jump in and answer that one for you.