What happens when a song is no longer under copyright?

Answer 1

If a song is no longer under copyright, then it becomes public domain.

Typically that means that the lyrics are no longer protected. Other people can use them for whatever reasons, commercial or otherwise, without needing to seek permission, licensing, or pay royalties.

That does not mean you can use someone else's cover of that song, which will be protected as a derivative work.

As an example, many Christmas carols were written a few hundred years ago, and their lyrics are no longer copyrighted. So you could do your own cover of 'I Saw Three Ships', publish and sell it, etc. But what you could not do is use someone else's cover, such as the one featured in a 1999 episode of South Park, which is protected by its own copyright until at least 2069, if not longer. You could not incorporate this version into a new work until then.

There are also different rules for copyright of the song and copyright of a recording made of a performance of the song. Each recorded performance may have completely different copyright owners and duration.