This is particularly interesting...
The narrator employs auditory imagery—imagery that describes something heard—when he says,
the sound of the inquisitorial voices seemed merged in one dreamy indeterminate hum. It conveyed to my soul the idea of revolution—perhaps from its association in fancy with the burr of a mill-wheel.
This is particularly interesting in that he connects something that one might hear with a description of something one might see, using visual imagery. He describes movement, the revolution or turning of a mill-wheel, in order to describe the way his inquisitors' voices sound. This combination of two different kinds of imagery, both visual and auditory, is called synesthesia.
More visual imagery follows when the narrator describes the appearance of his judges whose voices he's just described:
I saw the lips of the black-robed judges. They appeared to me white—whiter than the sheet upon which I trace these words—and thin even to grotesqueness; thin with the intensity of their expression of firmness
They wear black, then, but their lips are terribly, monstrously white and thin. They sound less like judges and more like creatures, something not human. This visual imagery continues when he describes what they look like when they speak:
I saw that the decrees of what to me was Fate were still issuing from those lips. I saw them writhe with a deadly locution. I saw them fashion the syllables of my name; and I shuddered because no sound succeeded.
Again, then, we have a synesthesia of visual and auditory imagery: the narrator describes the appearance of those monstrous lips speaking his name, but the silence that issues is equally as appalling. More visual imagery follows:
I saw, too, for a few moments of delirious horror, the soft and nearly imperceptible waving of the sable draperies which enwrapped the walls of the apartment. And then my vision fell upon the seven tall candles upon the table. At first they wore the aspect of charity, and seemed white slender angels who would save me, but then, all at once, there came a most deadly nausea over my spirit, and I felt every fibre in my frame thrill as if I had touched the wire of a galvanic battery, while the angel forms became meaningless spectres, with heads of flame
Thus, the narrator describes, using synesthesia, the look and sound of the draperies moving as well as the candles in the room—at first appearing good and lovely, and then inducing him to feel nauseated and terrified. They are, at first, angels, and then they transform into monsters as well, monsters with heads all aflame.
More auditory imagery:
And then there stole into my fancy, like a rich musical note, the thought of what sweet rest there must be in the grave.
And, finally, the combination of visual and auditory again in the final lines:
[the candles'] flames went out utterly; the blackness of darkness supervened; all sensations apepared swallowed up in a mad rushing descent as of the soul into Hades. Then silence, and stillness, and night were the unverse.
This description of the darkness (visual) and the silence (auditory) presents another example of synesthesia.