
Revealing mistake: After a close-up of the drawing of one-armed Lord Nelson, Killick opens the lid of the steaming food for Jack in a close-up, just before Killick says, "Oh, here we go again. Scrape, scrape, screech, screech." Later, after Pullings takes command of Acheron, Jack tunes up his violin and Killick says, "Here we go again," in another close-up of the steaming food for Jack. Every detail of the food shown in both close-ups is exactly the same - obviously shot at the same time. (00:26:25 - 02:05:20)
Chosen answer: They stop singing because they hear Hollom joining in. There are very strict ranks in the Navy, and Hollom singing with the lower crew dissipates these ranks, which is undesirable. That Hollom sings with the crew is frowned upon by the captain, as can be seen by his serious/agitated reaction after Stephen mentions that Hollom has such a clear voice.