The poem opens with alliteration, with the first line repeating the same word and, therefore, the same sound. The effect of this is noticeable right away, especially since the /br/ sound is quite prominent. There is a subtle power to this sound, since /br/ creates a low, rounded note that stands out on its own. Indeed, /br/ isn't the kind of sound that is easily overtaken. In this way, the repetition of /br/ aligns with the poem's imagery of waves rolling toward the shore, bringing a sense of strength to the opening line.
Further, each instance of alliteration in line 1 (and, for that matter, line 13) are separated by masculine caesuras (or caesuras that appear after stressed syllables). This places even more emphasis on the /br/ sound. This, in turn, gives the first line a forceful quality that once again reflects the might of the waves as they crash onto the shore. As a result, the speaker prepares readers to view the natural world as relentless and unstoppable—an idea that aligns with the speaker's feelings of powerlessness against the fleeting nature of existence.
There are a number of other alliterative moments throughout the poem, like when the /th/ sound repeats in the words "the" and "that" in line 4. The second stanza also features a smattering of /s/ sounds, creating sibilance that subtly evokes the hissing sound that waves make after having broken on the shore. In keeping with this, most of the alliterative sounds in the second and third stanzas (except for the /b/ sound in line 8) are soft, effectively mimicking the gentler sounds that otherwise strong natural elements like waves often make.
However, this softer, gentler form of alliteration vanishes in the final stanza, as the speaker not only repeats the blunt /br/ sound, but also employs a forceful /d/ sound in line 15: "But the tender grace of a day that is dead ..." As a result, the poem's overall sonic quality returns to a harsh, bleak tone that reflects the speaker's unhappiness.