Scott Fizgerald's short story "Afternoon of an Author" mentions "silent rights" and "sound rights" in regards to copyright.
Anyhow he had no more equity in that property — he had sold the silent rights many years ago and the sound rights last year.
As I understand this, he first sold rights "unofficially", "silently" without signing any papers. And last year he sold rights completely.
Or maybe "silent rights" means rights to make silent film (because main character seemingly sold rights to Paramount, so they could make a movie about his book) and "sound rights" means rights to make film with sound.
But all of this is just thoughts, I don't have any clue about what this means and searching doesn't help.