Because notification is not required for protection, and because protection is automatic, it's usually safest to assume materials you encounter are protected until you can prove otherwise. In the US,...
Printed music enters the public domain the same as most other materials; works before 1923 are in the public domain, and current (1978-present) works are protected for the life of the creator plus 70...
If you are looking for public domain sheet music, there are several sources linked below. If you are looking for public domain recordings, you will be more successful searching for Creative Commons...
For printed music, works published prior to 1923 are in the public domain, although newer arrangements or editions may still be protected. For recorded music, there is no way to be sure; the vast...