In the first and last paragraphs of "The Pit and the Pendulum," how are the "voices" similarly described?

Great question! I had never noticed this point before. At
the beginning of the tale, when the narrator receives his sentence, the voices are
described as follows:

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After that, the sound of the Inquisitorial voices
seemed merged into 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.

Of course, as you
correctly point out, voices are referred to again in the last paragraph, but under very
different circumstances:

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There was a discordant hum of human voices! There
was a loud blast as of many trumpets! There was a harsh grating as of a thousand
thunders!

It appears that the
voices in both cases refer to the forces of the Inquisition. At the beginning, note how
they all merge into one, "dreamy, indeterminate hum" reflecting the state of the
narrator but also the way that the Inquisition are able to decide on his sentence at
their leisure. At the end, however, with the invasion force, they are all set apart and
divided, thus explaining the "discordant" hum that clearly reflects the agitation in the
Inquisition.