Here are some sites that will let you host mp3s for free:

http://filelodge.bolt.com/index.php
http://megaupload.com/
http://studentslackers.com/media2/
http://www.garageband.com/upload
http://www.streamload.com/
http://www.mp3-host.com/

Here's a list of free file hosting sites (some of which allow mp3s):

http://www.free-webhosts.com/free-file-hosting.php

Hope this helps!