Coleridge starts us out with some heavy alliteration in the first stanza of "Christabel." The harsh, hard C sounds in lines 1-10 ("castle clock" and "crowing cock") are almost alarming, as if the speaker wants to make sure that we're awake enough to pay attention. The rest of the stanza slips into a series of SH sounds in lines 11-13 ("shine and shower"), lulling us back to sleep. This flip-flopping of harsh and gentle keeps us on our toes and reflects the ambiguity that we'll experience through the rest of the poem (see "Symbols, Imagery, Wordplay" for many examples of that).
Another important sound element in the poem is the hissing S sound, which mostly shows up when Geraldine is around. It sounds much like the hissing snake that Geraldine is supposed to represent. In fact, the word "hissing" shows up twice: first in stanza 47, and then again ten stanzas later. The word "hissing" is, of course, an onomatopoeia. It might also be worth noting that Christabel's name itself is pretty ambiguous, containing both a hard sound at the beginning and a nasty hissing sound right smack in the middle.
Overall, when read aloud "Christabel" has a sing-song quality that is almost hypnotizing, like a lullaby. It's as if the forces of good and evil are working against one another in terms of the very sounds of the poem. These harsh and soft sounds work their magic on the reader as well as the characters. We're lulled into sleepiness and vulnerability by the forces of evil while the forces of good try to snap us out of it with its sharp warnings.