They dared not make a movement or exchange a word that would have told the ghost that they knew that he was there! ...

What was going to happen?
This happened.
"Co-ack!" Their joint exclamation of horror was heard all over the house.

THEY FELT THAT THEY WERE SMARTING UNDER THE GHOST'S ATTACKS.
Leaning over the ledge of their box, they stared at Carlotta as though they did not recognize her.

That infernal girl must have given the signal for some catastrophe.

Ah, they were waiting for the catastrophe! The ghost had told them it would come! The house had a curse upon it! The two managers gasped and panted under the weight of the catastrophe.