Alliteration gives "A Slumber did my Spirit Seal" its musing, reflective tone, and draws attention to the contrast between the speaker's "slumber" and his awakened grief.
Strong sibilant alliteration appears in the very first line: "A slumber did my spirit seal." Repeated /s/ sounds often feel gentle and quiet; here, that quietness evokes the speaker's past life, when he metaphorically "slept" in blissful ignorance, with no "human fears" even crossing his mind.
Later, as the speaker tries to wrap his head around his beloved's death, the alliterative /r/ evokes not just a mood, but a landscape. The beloved's body, he says, is:
Rolled round in earth's diurnal course,
With rocks, and stones, and trees.
The rumbling /r/ sound recalls exactly what it describes: rocks rolling round. The repetition of the /r/ even suggests the constant cycling of "earth's diurnal course"—the globe's daily rotation. And the alliteration here connects to strong /r/ consonance, too: listen to all the internal /r/ sounds in "earth's diurnal course." These rough, insistent /r/ sounds contrast with the gentler /s/ alliteration in the first stanza, making it clear that the speaker has awakened to the hard reality of death.
A subtler alliterative sound here might go unnoticed in a longer poem—but in only eight lines, it catches the reader's attention. That's the repeated /f/ sound that connects "fears," "feel," and "force." While these words don't appear right next to each other, they're strong enough to stand out, and they link the poem's two stanzas together.
Each of these vivid words relates to something that's missing. In the first stanza, the speaker has no "fears," and imagines that his beloved can't "feel" the passing of time. In the second stanza, those illusions are gone, and the speaker has to reckon with the fact that his beloved now has "no force," no vitality. The emphatic /f/ sound thus tracks the speaker's transformation from a naïve young man to a grieving older one—and his beloved's transition from life to death.