When I started the Kow Cheng Tomb instance and the quest that comes with it as a Swashbuckler, I approached Moo Manchu and his dialogue states that he wouldn't have controlled the Stone Legion army of Kow Cheng without the Black Pearl, and he says "Arise, my Stone Legions! Crush them! Crush them all!". Afterwards, he says the duplicate line after "Fool! It is far too late to stop me now!" that is supposed to be meant for other classes. I say, for the Pirates who chose to be Swashbucklers, I recommend you kindly cut the duplicate dialogue line and have Sarah Steele's new dialogue line in place, saying that she knew she couldn't trust him all along and that when she's done with Moo Manchu, she'll destroy the Black Pearl once and for all before Old Scratch says his line "Here them come! Bang that Gong!". Thanks!

Er...I think this applies to everyone who had done three of the Kraken Skulls companions SECOND promotion quests, each one a slightly different thing.

SPOILERS BELOW

1. Swashbucklers - Sarah Steele must recover the Black Pearl to get her cousin back and give it to Moo Manchu.
2. Privateers - Lieutenant Emmett has to recover the Jade Jar of Kow Cheng in order to save the Marleybone-MooShu treaty.
3. Witchdoctors - Mormo helps defeat Moo Manchu's great enemy Dai Nao in order to lift his curse.

We also have two other encounters with Moo Manchu in the other promotion quests for different classes.

4. Musketeers - Wing Chun must recover the Egg of the Phoenix from Moo Manchu in order to become a Crane Master.
5. Buccaneers - Kobe Yojimbo must defeat a traitor to his noble house and reclaim his ancestral armor, and the traitor works for none other than (you guessed it) Moo Manchu.

Out of curiosity, have you completed the OTHER promotion quests on other pirates BEFORE doing the Tomb of Kow Cheng dungeon? If yes and the dialogue doesn't appear, I wonder why...it would also make sense for the mentioned MooShu Five companions to speak during the dungeon....hmmm.....