With his use of alliteration, rhyme, and internal rhyme, Poe can also control the mood of the poem. In addition to all the eerie clues given to us by the setting -- a "midnight dreary" in "bleak December," following the death of the narrator's lover -- the sound devices he employs are a more subtle way of impacting us.
For example, the "s" alliteration in the line about the curtain (cited by the other commenter) is followed by a line discussing the effect of the curtain's rustling on the narrator: he says, it "Thrilled me -- filled me with fantastic terrors never felt before." The alliterative repetition of the "f" sound is similarly soft, but it sounds even more like the wind that might blow the curtain rather than the curtain's rustling itself. It's as though, in the silence after the strange tapping at the door, the only noise to be heard is the eerie wind rustling the curtain. Such soft sounds almost intensify the silence that exists otherwise.
Later in the poem, the narrator describes the raven as a "grim, ungainly, ghastly, gaunt and ominous bird of yore." The alliterative "g" in this line is a much harsher sound than the "f" and "s" of earlier in the poem. The narrator is disturbed by the bird that has entered his room, and his repetition of the guttural, hard "g" sound seems to be an indication of his perturbation.
Further, the internal rhyme and end rhyme are fairly predictable in the poem. As such, the reader gets used to hearing the rhymes and begins to anticipate them, lulled into a sense that everything is regular in the poem. However, this regularity of rhyme juxtaposes sharply with the irregularity of the poem's content: what happens to the narrator is most certainly not predictable or regular. This disconnect is off-putting for the reader -- we are simultaneously lulled by the rhymes and made nervous by their meanings. This enhances the poem's eerie mood as well.
The repetition comes across as very spooky, and the rhyme gives the poem a sing-song quality that is also very haunting. The words are carefully chosen to make the reader uneasy.
There is a musicality to the trochaic octameter and the other rhythmic devices such as onomatopeia, alliteration, assonance, parallelism, and rhyme of Poe's "The Raven" that seems almost Baroque in its recurring motifs.
The sound techniques appearing throughout the poem are numerous and sometimes complex. The use of end rhyme, internal rhyme, the interlocking rhyme scheme, and alliteration creates the effect of lulling readers and drawing us into the fantastic supernatural tale.
The alliteration, in particular, creates an effect that is almost hypnotic, as seen in this phrase: "While I nodded, nearly napping." Poe's frequent repetition also contributes to the poem's hypnotic effect. These devices are especially effective in the poem's beginning as Poe establishes the mood.
The examples of onomatopoeia in the poem contribute to our sense of the setting. They serve to put us there with the narrator in that room with the fire dying and shadows looming. One good example of this effect of onomatopoeia would be this line from the third stanza: "And the silken, sad, uncertain rustling of each purple curtain . . . ." With these words, we can hear the curtains barely moving, breaking the ominous silence.
Poe's sound techniques in "The Raven" accomplish these several effects, while enriching the poem's tone.