Some creative works (for example, music) are eligible for copyright protection for a limited (but ridiculously high) number of years. After that time, they are considered to be owned by no one, and anyone can do anything they want with the things! When this copyright expires, the music becomes Public Domain.

What if you want to write new music and have no copyright attached to it? This is not common. In Europe, for example, it seems to be impossible to start a creative work with no copyright. The EU considers the copyright of a creative work to be an inalienable right, and any contract or agreement severing that right will be deemed invalid.

If any site says they have real Public Domain Music - be very careful! They may believe they have real new Public Domain music, but I would need to see some proof. You can't just "declare" something is Public Domain.

Similar claims of Copyright Free music are also unlikely. Something is free of copyrights either 1) when the copyrights expire... or 2) if it was not eligible for copyright in the first place. Music recordings are eligible for copyright**. It takes a long time for those copyrights to expire. Not hours or years, but generations.

Please consult your local Copyright Lawyer for more details.

**In this context, I mean a sufficiently original work, intentionally recorded.