Alliteration is used sparingly in "The Wild Swans at Coole," but to great effect. Line 4, for example, alliterates in the phrase "still sky." The focus on this hushed /s/ sound momentarily quiets the poem, matching with the phrase's idea of calm stillness.
The next important example doesn't come until just over halfway in the poem—in fact, two examples come close together:
The bell-beat of their wings above my head,
Trod with a lighter tread.
"Bell-beat" describes the effect of the swans' wings on the air as they flew over the speaker during his first visit to Coole. The alliterative /b/ sounds have a muscular rhythmic quality, mimicking the strength and sound of the birds' wings. Shortly after, as if subconsciously wanting to imitate the swans, the speaker alliterates in a description about himself. The play of "Trod" with "tread" has a nursery-rhyme or riddle-like sound, convey the way that the speaker used to walk more lightly. On his first visit to Coole, he was more optimistic about life, and, of course, younger. Now he feels the heaviness of his steps.
The next meaningful example comes in line 19. In this stanza, the speaker strongly personifies the swans:
Unwearied still, lover by lover,
They paddle in the cold
Companionable streams or climb the air;
The repeat of "lover" here—which is also diacope—has an almost luxurious sound, hinting at the pleasures and joys of being in love (which the speaker seems to mourn for). Note that this effect from the /l/ sound is further boosted by consonance in these lines ("paddle," "cold," "companionable," and "climb"). The repeated hard /k/ sound again focuses readers' attention on the swans' actions.
Line 23 also uses alliteration effectively:
Passion or conquest, wander where they will,
The three /w/ sounds seem to do their own wandering through the line, evoking the swans' freedom and grace of movement.