Some written works are in the public domain by nature, particularly published or unpublished works created by the U.S. federal government.
"A work is a U.S. government work if it was prepared by an officer or employee of the federal government as part of that person's official duties. 'Officers' include all elected and appointed officials of all branches of the U.S. government--for example, the president, members of Congress, cabinet members, and judges, as well as lower-level U.S. government officials and members of the federal bureaucracy." (Fishman 2004, 3/13).
Written works that are subject to copyright may enter the public domain when certain conditions are met. At present, the relevant conditions are as follows (Elias and Stim 2004, 122):
Works that do not meet one of these conditions should be assumed to be under the protection of copyright until proven otherwise.
Although sound recordings made before 1972 are not protected by federal copyright law, they are protected by various state statues and state common law copyright. Since early 1972, sound recordings are also covered by federal copyright law. (Fishman 2004, 4/44-4/45). Because separate rights arise for the various authors of a sound recording, including the content creator, the performer, and the recording producer (cf. Leaffer 1999, 136-37), sound recordings of speeches--even speeches the texts of which are in the public domain--should be assumed to be protected by copyright of some form or other until proven otherwise.
The main exception to the foregoing is the corpus of sound recordings created by U.S. government employees (or non-employees working for the U.S. government).
Audiovisual works became the subject of federal copyright law in 1976. In general, audiovisual recordings of speeches--even speeches the text of which are in the public domain--should be assumed to be protected by copyright of some form or other until proven otherwise.
A principal exception to the foregoing is the corpus of audiovisual recordings produced by employees of U.S. government as part of their jobs.
Fishman, Stephen. The Public Domain: How to Find & Use Copyright-Free Writings, Music, Art & More. 2nd ed. Berkeley, CA: Nolo, 2004.
Elias, Stephen, and Richard Stim. Patent, Copyright, & Trademark. 7th ed. Berkeley, CA: Nolo, 2004.
Marshall Leaffer, Marshall. Understanding Copyright Law. 3rd ed. New York: Bender, 1999.