Google is good for so many things, among which is searching for all sorts of files, including MP3's. Here's a quick primer:

• -inurl:(htm|html|php) intitle:"index of" +"last modified" +"parent directory" +description +size +(wma|mp3) "Nirvana"

Just substitute the term "Nirvana" for any band or singer you might be looking for, and your search will lead you to open indexes that contain downloadable music files.