"How soon hath Time" makes frequent use of alliteration. For example, in the poem's first 5 lines, Milton establishes a strong chain of repeated s sound (which is more specifically an example of sibilance), which appears in "soon," "subtle," and "stol'n." He also creates a repeated f, b, and T sounds throughout. The alliteration at times suggests an underlying connection between rather disparate concepts throughout the poem, such as time, theft, flight, youth, and spring.
In the second half of the poem, one is struck by the more direct and immediate connections between alliterating words: for example, "grace" and "great" in lines 13-14. Here, it is God's grace that makes Him great: it is natural and even elegant that these two conceptually linked words should also be related sonically. This conceptual and sonic connection is also evident in words like "still," "strictest," "soon," and "slow," which all generally relate to the idea of motion and control.
The poem thus embodies its argument in sound. When the speaker is struggling with anxiety and confusion, the alliteration becomes confused, full of questions; it's unclear how . When he finds his way toward certainty, the alliteration becomes smooth, elegant, and appropriate.