Plath uses alliteration throughout the poem, which makes its feelings of heartbreak and despair seem all the more intense. For example, note the hammering /d/ sounds that appear in one of the poem's two refrains:
I shut my eyes and all the world drops dead.
These heavy, thudding /d/ sounds call attention to the phrase "drops dead" while also sonically mimicking the very "dropping" sensation being described.
Similarly, in the next line, gentle /l/ alliteration highlights the image of the speaker opening her eyes and returning to the world:
I lift my lids and all is born again.
The lilting sounds have a delicate and airy feeling, akin to eyelids fluttering open.
The alliteration is particularly intense in the poem's third stanza, where it overlaps with the related devices of consonance and assonance. The bold /b/ sounds here link "bewitched" and "bed," in turn linking madness or trickery with intimacy. In the next line, a flurry of repetitive sounds essentially turns up the poem's volume, helping to convey the intensity of the speaker's emotions in this dream. There's alliteration ("me," "moon"; "kissed," "quite") as well as hissing sibilance, sharp /k/ consonance, and blunt /uh/ assonance:
And sung me moon-struck, kissed me quite insane.
Altogether, the line sounds bitter, frantic, and emotional. The intensity lingers into the next stanza as well, with the alliteration of "fires fade." That /f/ sounds pops up again in "Seraphim," while /s/ alliteration links those angels to "Satan's men." The fricative, hissing sounds bring to mind a fire being snuffed out.