'Public Domain' is a designation for works whose copyright period has expired or which have never been copyrighted. Works published before 1 January 1928; works published without a copyright before 1 January 1977; works published before 1 January 1989 if the author(s) did not register a copyright within 5 years; and works published before 1 January 1964 without renewing copyright are not protected by copyright as of 1 January 2023. An interesting exception to copyright is works prepared by a US government personage in the course of their official duties, as those works are classified as 'Public Domain' regardless of classification.
However, multiple categories of works not subject to the above periods exist. If the work was never published, it is protected by copyright for seventy years after the demise of the author(s). Works completed by the author as an aspect of employment are protected by different rules, and the copyright holder might be the employer rather than the author.
Sound recordings are covered by the 1971 Sound Recordings Act and the 1976 Copyright Act. Any sound file recorded before 15 February 1972 are covered by (as applicable) state laws and common law. Those protections will expire on 15 February 2067, at which point all of those recordings will be classified as 'Public Domain.'
Those protections were modified with the passage of the Music Modernization Act in September 2018. Any works recorded before 1923 are already public domain; works recorded 1923 - 1946 are protected by copyright until 100 years after publication; works recorded 1947 -1956 are protected for 110 years after publication; works 1957 - 14 February 1972 are protected until 15 February 2067; works 15 February 1972 - 31 December 1977 are protected for 95 years after publication; works recorded after 1978 are protected for 70 years after the death(s) of the author(s) or for 95 years after publication in the case of works with corporate ownership.
Video recordings are protected by copyright for 95 years. The period runs annually from the first of the year, regardless of at which point in the year the copyright for the film was filed. However, as with audio recordings all video recordings made by US government employees in the course of their official duties are considered public domain immediately.
The Uruguay Round agreements of 1996 created a legal dilemma, which was addressed in contrary ways under various court rulings. This legal quandary currently results in a more recent dubbed, subtitled, or restored version of a work in public domain is eligible for its own modern copyright. Similarly, additional features or packaging are currently eligible for their own copyrights. As the legal understanding of 'copyright' is continually evolving and under legal argument, particularly if/when a digitally remastered edition of a public domain work is the subject of legal action, questions of copyright involving currently public domain videos do not yet have firmly settled answers.
