Music presents its own particular copyright challenges.  Some of the following sources may help:

A Guide to Copyright for Music Librarians: http://copyright.musiclibraryassoc.org/

Public domain music

• Public domain music list:   http://www.pdinfo.com/Public-Domain-Music-List.php
• IMSLP Petrucci Music Library (free public domain sheet music library):  http://imslp.org/wiki/Main_Page

Searching for Music

• ScorSer (search system for musicians):  http://en.scorser.com/D/Everything.html

Copyright Registration of Music:  http://www.copyright.gov/fls/fl105.html

The Educational Cyberplayground has a page with a substantial body of links and is devoted to Music Copyright, although it may not be as sophisticated as some might need.  Music and Copyright Law:  http://www.edu-cyberpg.com/Music/musiclaw.html