Found this on www.lifehacker.com

Use Google operators to find unprotected directories of audio files or files of type .mp3. As suggested by this Google Tutor & Advisor article , a Google search like this:

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

Turns up several directory listings of Flaming Lips MP3's. Replace "flaming lips" with whatever you are looking for. Works like a charm