Very few sound recordings are in the public domain in the United States; in fact, most sound recordings will not enter the public domain until February 2067! However, there are many sound recordings that have been licensed with Creative Commons. See the Creative Commons page for resources.
When you take a photograph, write a poem, or even jot down a to do list, your work is automatically protected under U.S. Copyright Law, and people will usually need to ask your permission to share your creation online, translate it into new languages, or perform it in front of an audience. Well into the future, the copyright will expire, as it already has for millions of works of art, music, film, and literature. These materials have become part of the "public domain," collections that are legally available for anyone to reuse or remix.
An item might be in the public domain in the U.S. if:
It was published before 1926. (Note that published in this context means “copied and distributed beyond a small group of people.” This means flyers, brochures, etc. are usually considered “published.”)
It was published from 1926-1977 but there is no © symbol or statement printed on the item.
It was created before 1901 by an anonymous creator but never published.
The item was unpublished (e.g., a family snapshot) and the creator died before 1951.
The item was created by the Federal government or Florida government employees.
Anyone who owns a copyright may also share their work as part of the public domain, meaning others may legally use it without permission. Usually this is done by labeling with a "CC0" license; this and some other licenses allow for almost any use, commercial or noncommercial.
It's important to remember that just because it is legal to use something, this doesn't always make it right. There's a complex history behind every cultural work, and even if the copyright has expired sharing the material could invade someone's privacy or appropriate aspects of someone else's culture. Many works in the public domain were created in the 19th or early 20th centuries and contain racist imagery that continues to be harmful and potentially traumatic today.