Pubic domain occurs naturally when a copyright expires. In theory, a copyright owner can intentionally disclaim any power to enforce a copyright, making it as if it were "public domain".
Much of Rachmaninoff's work was composed prior to 1923 & should have already entered the public domain. The balance of his work which still falls under copyright protection will be copyright free...
In terms of intellectual property "public domain" refers to items that have no restrictions of use due to expiration of legal controls (patents, trademarks, copyright)
"Over the Rainbow" sung by Judy Garland for the movie "The Wizard of Oz" (music by Harold Arlen and lyrics by E.Y. Harburg) was published in 1939. According to the U.S. Federal Copyright Act of 1978...
There are only two Mr Ed episodes that I've been able to confirm that are in the public domain. ""The Wonderful World of Wilbur Pope" and "Wilbur Gets the Message... About Savings Bonds". However...