mp3.com is shut down, better tell your friends to put their audio on their own site.
not sure how many of these sites where unsigned bands can be heard. Here's one I found and listened to about 500 bands/artists on - what I found is there is a very good reason they are unsigned - NO TALENT!
if you can't write a song yourself or find someone to write for you it doesn't matter how good a musician you are - the music business still revolves around the song - whatever genre you're in.
http://www.vitaminic.com