There's an easy way to find free MP3 files with Google; actually, there's quite a few easy ways. Here's one (this particular method via Tech-Recipes):
-inurl:htm -inurl:html intitle:"index of" "Last modified" mp3
Okay, so let's break this down. You are attempting to find directories, so you don't want a a Web page (hence the minus sign in front of both the htm and html extensions). You're looking for the root directory, so that's where the "index of" in the title (intitle) comes in. And that mp3 file designation? You can change that to any file you are wanting to look for: wma, pdf, etc.
Now, how about looking for specific artists? Easy:
-inurl:htm -inurl:html intitle:"index of" mp3 "beethoven"
Make sure you put the artist's name in quotes at the end of the search string. Now, there's another way to do this that might be a little bit easier; you're just changing the order of your search query around.
intitle:index.of + mp3 + "beethoven" -html -htm -txt
Basically, you're doing the same thing that you did in the first search string. You can narrow it down even further if you want to:
"index of" + intitle:mp3 + beethoven
All you're doing here is looking for directories with MP3 in the title and Beethoven.