Use Google operators to find unprotected directories of audio files or files of type .mp3.
-inurl:(htm|html|php) intitle:"index of" +"last modified" +"parent directory" +description +size +(wma|mp3) "Nickelback"
This will turn up several directory listings of Nickelback MP3’s. Replace “Nickelback” with the artist, genre or keyword of your liking.
Note: this is not intended to be used to download mp3’s illegally.