How does Montresor's response to Fortunato screams add to the mood as the story reaches its high point?
I assume you mean the near-end of the story:
A succession of loud and shrill screams, bursting suddenly from the throat of the chained form, seemed to thrust me violently back. For a brief moment I hesitated -- I trembled. Unsheathing my rapier, I began to grope with it about the recess; but the thought of an instant reassured me. I placed my hand upon the solid fabric of the catacombs , and felt satisfied. I reapproached the wall. I replied to the yells of him who clamoured. I reechoed -- I aided -- I surpassed them in volume and in strength. I did this, and the clamourer grew still.
Montressor had been quiet and in a state of denial regarding Fortunato's motives until then, so the screams reveal his realization that he is indeed the victim of revenge, that his death is near.
Poe uses this passage to show how focused and sadistic his speaker is. Not only does Montressor scream, but the narrator screams even louder. Not only does Fortunato get physical revenge on his victim, but he gets verbal revenge as well: his words and screams silence his victim.
The passage reflects the gothic mood of the story: death and decay are inevitable; our shrieks from the grave will never be heard.