Audio problem: After releasing the whaler prisoners aboard Acheron, Mr. Hogg shouts, "Now do your worst!" though his mouth clearly does not say that. (01:56:25)
Audio problem: While doing repairs on Surprise, Jack says, "Repair won't do here. I need those replaced," but his lips don't move when his voice says, "..replaced." Then as he says, "Ask Mr. Lamb what he has in stock," his mouth actually says something else. (00:22:55)
Audio problem: After the Acheron attack on the Surprise, below deck Jack, Stephen and some others discuss the situation. When Mr. Allen says, "Triple-shotted at 200 yards, and our guns had no effect," his mouth is actually saying, "...at 500 yards..." (00:18:20)
Audio problem: Jack tells Pullings that he will take the next watch. Pullings says, "Aye, sir. Mr. Mowett, Mr. Allen, calmly now. You know his orders," however he does not say the latter half of that sentence, he says something else. (00:42:55)
Audio problem: The last scene, when the captain and the doctor are playing music, the captain starts out with playing the violin with his fingers. Then the doctor is playing light tones with the bow on his cello. Suddenly, they switch, and the doctor is using his fingers, playing deep tones on the cello, and the captain is now playing with a bow. Finally, the first part of the music piece come back, and you hear both a violin (or some other small string instrument) played with fingers, and another violin played with a bow.
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.