I did a search and found a MP3 virus called BLOAT but it looks to have been a hoax...

http://www.mp3.com/news/111.html

Mp3 files do not have execution code, and even if they had some high level execution code (like Java), it would not be given full access to your system because that would be way to excessive for a music file.

What might have happened with that file your downloading was that, norton just looks for patterns, if you recall you said the file extension was "dl" not "mp3" (similar to DLL but that is not the issue), viruses have certain kinds of patterns and some like to hide themselves in data files, that kind of virus has to be executed to run in your system (some other program has to know where the virus is to execute it and mp3s again dont have executable code). So what likely happened is that coincedently a pattern in the mp3 file looked like a viruses pattern to norton.

Just in case though, I will do a little more research on MP3 viruses to see if there was any other hoaxs or any other issues...