Lets say you want to find MP3 songs from Madonna, basically you can just type in this search pattern:
"index of/" "madonna" "mp3".
This way works pretty well and it’s also the way I used to search for MP3 files. But if you use this pattern, in most cases the search result convince more:
-inurl:(htm|html|php) intitle:"index of" +"last modified" +"parent directory" +description +size +(wma|mp3) "madonna"
Try it out! You’ll see how it works…