lamarca, you wrote:

When a song is written, who holds the copyright in most cases? The lyricist? The librettist (if the two are different people?) The recording company that releases the first recording of it? The performing artist who made that recording? The performing artist(s) who makes a commercial hit of it? The recording company who published the hit recording? The publisher of the sheet music?

My understanding of the U.S. situation is (but the usual disclaimers apply: I am not a lawyer, this is not legal advice, etc. etc.) as follows:

When an original song is first written, the law creates a copyright in the lyrics and grants it to the lyricist , and it creates a copyright in the music and grants it to the composer. These are two separate copyrights in a case of this kind. As I mentioned earlier, the copyright is sometimes assigned to a publisher. If it is, the publisher becomes the new owner. In theory the original author can "terminate" the assignment after 30 years and recapture the copyright. In practice this is not easy to do, and the termination mechanism is little used. There is a link from the "Should Auld Copyrights be Forgot ?" thread to some news articles about termination of half the copyright to Superman (half, because in that case the two authors were joint authors of the whole work, each owning half the copyright interest in the single whole), if you want to know more about termination rights.

T.