Visually, Fortunato looks like a drunken clown. He is dressed for the Carnival. He has the appearance of a court jester with the "conical cap and bells." Montresor lures Fortunato to his catacombs by appealing to his pride of being a connoisseur of wines. Montresor is quite successful in playing to Fortunato's pride. He also keeps feeding him wine as they go, keeping Fortunato drunk. The image of Fortunato as a jester or a "fool" illustrates how easily he is fooled by Montresor.
Montresor, himself, dons a black mask. He does this to disguise himself as he leads Fortunato from the Carnival. The mask also symbolizes death, an example of foreshadowing that we might attribute to Montresor as well as Poe.
As they go farther into the vaults, the climate becomes colder, with moisture and nitre on the walls. The farther they go, the darker and colder it becomes. The darkness is associated with death. The cold symbolizes the coldness of a corpse and Poe plays with this idea of Fortunato's cold/cough. Fortunato claims that he will not die of a cough. Montresor agrees, knowing he will die another way.
When Montresor walls Fortunato in, he hears Fortunato laughing, thinking this is a practical joke. But Fortunato's cries become more and more desperate, heightening his desperation, and ending with "For the love of God, Montresor!" And finally, all Montresor hears is the jingling of Fortunato's bells. Fortunato has ceased crying out. All he can do is consider the tomb he is in. The sound of the bells by themselves is haunting, as if to say that Fortunato no longer has a voice.