Creative materials that are not protected by intellectual property laws like copyright or trademarks are in the public domain. The public owns these works, not an individual author or artist. Anyone can use a public domain work without permission.
There are four common ways that works arrive in the public domain:
Any Song or Musical Work Published in 1922 or earlier is in the Public Domain in the USA. Cornell University has a comprehensive list of when works are available in the public domain.
No Sound Recordings are Public Domain in the USA due to very complex Federal and State Laws. What does that mean? You can find free music to use if available with a Creative Commons License but other sound recordings will not be available in the public domain. However song or music (think lyrics and sheet music) can be available in the public domain. So if you are musically inclined you can create a sound recording using the music available in the public domain.