"Adam's Curse" is a highly musical poem that shows off Yeats's ability "to articulate sweet sounds together," as the speaker puts it in line 10. One of the musical devices the poem uses is alliteration, which makes its language more harmonious and pleasant. The alliterative phrase "sweet sounds" is a perfect example of this kind of verbal "sweetening"!
Notice, too, that alliteration increases in lines 21-26, as the speaker describes a traditional, chivalrous style of courtship. As part of this courtship, lovers used to quote "Precedents out of beautiful old books"—in other words, beautiful passages of classic literature. Alliteration helps the language in these lines rise to a similar level of grandeur:
I said, ‘It’s certain there is no fine thing
Since Adam’s fall but needs much labouring.
There have been lovers who thought love should be
So much compounded of high courtesy
That they would sigh and quote with learned looks
Precedents out of beautiful old books [...]
But the alliteration disappears in the next line, as the speaker disparagingly describes modern love. Partly as a result of these sound effects, this line sounds blunt and anticlimactic.
Alliteration also adds force to the phrase "daylight die" (line 29), making the "death" of the light seem more dramatic. (Symbolically, it reflects the passing of the speaker's youth, much like the "summer's end" in line 1.)
The repeated /w/ sounds in "Washed by time's waters" (line 32) evoke the wavelike motion the speaker is describing. Repeated /w/ sounds also combine with /h/ alliteration in the final two lines:
That it had all seemed happy, and yet we’d grown
As weary-hearted as that hollow moon.
These soft consonants add a quiet, wistful lyricism to the poem's unhappy ending.