What can you conclude from the lines: "There came forth in return only a jingling of the bells. My heart grew sick--on account of the dampness of the catacombs"?
Montresor calls to Fortunato twice and receives no answer.
No answer still. I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells. My heart grew sick; it was the dampness of the catacombs that made it so.
These words are hard to understand because we cannot tell whether the jingling of the bells was caused by the torch hitting Fortunato's hat or whether he was shaking his head. It seems probable that Poe wanted to indicate that Fortunato was still alive but shaking his head in despair and refusing to be tormented by Montresor any further. Another possible interpretation is that the torch fell on Fortunato and he had to shake it off to keep from being burned.
As far as the sentence about Montresor's heart growing sick, the reader must realize that Montresor is momentarily horrified and perhaps even regretful for what he has done to another man. Montresor does not want to acknowledge this even to himself, so he rationalizes: he attributes his feelings to the dampness of the catacombs.
Poe makes many references to the dampness of the catacombs. His intention seems to be to assure the reader that Fortunato will not die of thirst. He will lick water off his own hands and off the granite wall to which he is chained. This makes his situation all the more horrible, because he will have to die by starvation. A man might live without water for a week, but he can survive for a month without food. Part of the horror of this story is evoked by the thought of what it will be like for the prisoner to die standing up in the darkness, wasting away for lack of food, and hoping against hope that someone might find him--even that Montresor might have a change of heart and come back and release him.