Materials in the Public Domain
What is Public Domain?
Mirriam-Webster Online defines it as:
"the realm embracing property rights that belong to the community at large, are unprotected by copyright or patent, and are subject to appropriation by anyone."
Basically, if something is in public domain you are free to use it in any way you would like, however determining if something is in the public domain is sometimes difficult. Also, there is a difference between books and scores (or print material) and sound recordings.
A general rule of thumb is that most scores published 1922 or before are most likely in the public domain. The University of North Carolina, Chapel Hill has a chart that can help you with the basic determination: http://www.unc.edu/~unclng/public-d.htm. The Public Domain Information Project is another very useful site. It includes lists of songs that have moved into the public domain, a list of resources that can be used to prove something is in the public domain, Links to legal and copyright-related sites for further research, and an extensive list of sites with materials that are copyright free.
As far as sound recordings, it is generally accepted that there are none in public domain and there will not be until 2067, due to the Sonny Bono Act. There is an explanation of the act in layman's terms on the Public Domain Information Project site mentioned above.