It says: The first is gone the rest will go unless you leave the island and row! row! row! Signed, the Phantom Shadow.
11 seconds sound clip from Scooby-Doo, Where Are You! - A Night of Fright Is No Delight (S1E16).
You can hear this line at 00:07:27.653 in the Blu-ray version of the episode.
Quote context
[...]
- Come on. Let's get some sleep.
- I don't know, but it came from Cousin Simple's room.
- Poor Cousin Simple. All that's left of him is his nightcap.
- Tsk tsk tsk. Isn't that a shame?
- Hey, get a load of this writing on this dusty mirror.
- Phantom Shadow? Then you were right, Scooby.
- Uh-huh.
- I think we better return to our rooms and lock our doors.
- Fantastic. That dummy looks just like Scooby-Doo.
- Do you think this plan will work, Freddy?
- Sure.
[...]