The issue of copyright and what is or is not in the public domain seems to come up with some regularity. Here is a recent article on what is now in the public domain as George Gershwin's composition "Rhapsody in Blue," along with most other works that were first published in 1924, entered the public domain in the U.S.
See The Murkiness of the Public Domain
Here is a handy checklist that is cited at the end of the article:
Copyright Term and the Public Domain in the United States