Montresor re-echos and surpasses his victim's desperate cries for help in order to show Fortunato that his cries are useless. By doubling the volume of the noise being made, Montresor demonstrates that the catacombs, deep underground and surrounded by granite, are virtually soundproof. Montresor also gets sadistic pleasure out of adding to Fortunato's torment in this way. He is showing his poor victim that he already thought of everything. The trap is perfect. Fortunato's last hope is futile. Nobody can hear him now, and nobody will be able to hear him after Montresor leaves him to die alone in agony.
This incident also demonstrates to the reader the same thing that is demonstrated to Fortunato. As Montresor explains:
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 re-echoed, I aided, I surpassed them in volume and in strength. I did this, and the clamourer grew still.
Edgar Allan Poe uses dramatic action and vocal effects to show Montresor chose the ideal location to commit his murder. The nonexistent cask of Amontillado has lured Fortunato into an inescapable death-trap. Poe probably thought it important to show that calling for help would do Fortunato no good, regardless of how long and hard the desperate man screamed and yelled. If the reader thought Fortunato might escape by yelling for help, now or in the future when he was all alone, Montresor effectively disposes of that possibility.