Items in the public domain are those that are not under copyright, usually due to their age. Because there have been many changes in copyright laws, it can be difficult to determine whether something is in the public domain.
Sound recordings created in the U.S. are protected under two separate copyrights: that of the composition, and that of the sound recording of the composition. So remember, when researching copyright protection of a sound recording, you must check the copyrights status of both the recording and the underlying composition.
What's in the Public Domain?
Sound Recordings in Foreign Countries
Keep in mind that sound recordings created and published in a foreign country are protected in a different manor. These protections are complicated, but in general, any recording created or published in a foreign country on a date that would result in it being in the public domain in that country as of January 1, 1996 is now in the public domain.
For more information see Copyright Term and the Public Domain from the Cornell Copyright Information Center
The following resources are great places to read more about public domain materials and usage.