USA Copyright Law for Sound Recordings
Sound Recording Rule of Thumb:
Songs and Musical Works have always been protected under U.S. federal copyright law. But sound recordings and records were so new and rare in the early 1900s, they were not included in the Copyright Act of 1909. Until 1976, sound recordings were protected by a hodge-podge tangle of state, county, and city laws. Many state and local laws included sound recordings under common law, which meant that ownership was forever and, unlike copyright protection, ownership of a sound recording would NEVER expire.
The Copyright Act of 1976 created a copyright category called Sound Recordings that now provides federal copyright protection for CD's, MP3's, WAV files, records, and other music recordings made after February 15, 1972. These Sound Recordings receive copyright protection for 70 years after the death of the last surviving author. But the Copyright Act of 1976 and the Sonny Bono Copyright Extension Act left copyright protection for sound recordings fixed or published before February 15, 1972, remaining under state law until 2067.
The Earliest that Copyright Protection will Expire for any Sound Recording in the USA is 2067
Remember that a musical work and a sound recording of a musical work have separate copyright protection. The children's song, "Mary Had A Little Lamb" is absolutely in the public domain worldwide, and it can be freely used by anyone. However, in the USA, no sound recordings of "Mary Had A Little Lamb" are in the public domain. If you want to use any sound recording - even a sound recording of a public domain song - you must either license a recording or create your own recording.
Sure, there are always a few exceptions to every rule. The U.S. government cannot hold any copyrights, so sound recordings made by the federal government can potentially be in the public domain. Some individuals have made sound recordings and placed them in the public domain. And because state law is so varied, there are pre-1972 recording that might be PD in one state or one county or one city. But even the experts can rarely be absolutely confident that they have identified one of the extremely rare PD sound recordings. So unless you have a law degree and Ph.D. in Intellectual Property, it is highly unlikely you will be able to competently recognize a public domain sound recording.
Here is the root of all the complexity and problems: The U.S. Copyright Act of 1976, § 301(c):
With respect to sound recordings fixed before February 15, 1972, any rights or remedies under the common law or statutes of any State shall not be annulled or limited by this title until February 15, 2067. The preemptive provisions of subsection (a) shall apply to any such rights and remedies pertaining to any cause of action arising from undertakings commenced on and after February 15, 2067. Notwithstanding the provisions of section 303, no sound recording fixed before February 15, 1972, shall be subject to copyright under this title before, on, or after February 15, 2067.
There are several U.S. websites claiming that sound recordings made in the United States prior to February 15, 1972, are in the public domain, and there are links to U.S. Copyright Office publications stating: "Sound recordings fixed before February 15, 1972, are not eligible for Federal copyright protection." It is quite true that pre-1972 sound recordings have no federal copyright protection, but they are still well protected under state law.  Virtually every sound recording in the USA is copyright protected at least until the year 2067.