When Fortunato sobers up and realizes what is happening to him—which is that he is chained up and in the process of being walled into a damp, dark, isolated niche in a catacomb to die—he begins to scream. Montresor joins him in screaming, finally drowning out his screams. As Montresor states:

I replied to the yells of him who clamoured. I re-echoed, I aided, I surpassed them in volume and in strength. I did this, and the clamourer grew still.

This a cinematic moment. We can picture Montresor lit by wavering torchlight in his black silk mask, amid human bones, almost finished with the wall in front of Fortunato. We can hear Fortunato's "shrill" screams and Montresor's screams in response, echoing in the silent space.

Why Montresor does this is open to interpretation, but given that he will soon after say he felt heartsick, one potential meaning is that he is interested in more than mocking and triumphing over his enemy. It may be that Fortunato's desperate screams are touching his conscience and his heart, and therefore, he screams over them to shut out any notions of compassion that might lead him to take pity on his victim.