Let us remember that the strong consistent rhythm is not the only sound effect that impacts the reader strongly when reading this poem or hearing it. Poe also uses alliteration and internal rhyme alongside the strong rhythm to relentlessly arrive at the grim conclusion of the poem. Working together, these three aspects certainly contribute towards a very dark, pessimistic tone that perfectly corresponds with the feelings of the narrator. As you read or hear it, you get the picture of the narrator being driven into an even greater frenzy as he desires to know the truth about the questions he asks, and the use of internal rhyme combined with the repetition of certain words and alliteration alongside the rhythm seems only to accelerate the speed of this poem and to help consolidate this image as we picture a man speeding up to run over the edge of a cliff. Consider the following example:
"Prophet!" said I, "thing of evil!--prophet still, if bird or devil!
By that Heaven that bends above us--by that God we both adore--
Tell this soul with sorrow laden if, within the distant Aidenn,
It shall clasp a sainted maiden whom the angels name Lenore--
Clasp a rare and radiant maiden whom the angels name Lenore."
Quoth the Raven, "Nevermore."
You hopefully have observed the repetition of words such as "maiden" and also examples of internal rhyme such as "evil" and "devil" in the first line and "laden" and "Aidenn" in the third, which of course rhyme with "maiden." Also note the alliteration in the penultimate line in "rare and radiant." It is very clear that the impact of all of these effects, including the very strong and consistent rhythm, is something that manipulates the reader and gives us a sense of the narrator's mind careering out of control as the relentless pace of the poem seems only to accelerate.