I've done a band system very close to your specification on
my site, but it's not released yet (probably tomorrow - we're releasing lots of things at once, so even though it's complete, other things aren't). The difference is that songs aren't linked to off-site MP3s - the band script finds all songs by the artist in the database and links to them via a media player on my site. Within the ACP, you add song information along with the link to the web address with the file.
I wouldn't want to give this away for free right now though, since I've put a hell of a lot of my free time and effort into making it. I'd also like it to be unique to my site.
You can take a look at the finished product when it's released and if you're really interested I can discuss it with my fellow admins.