"Nine Gold Medals" uses alliteration to ramp up the poem's tension or drama. Take line 10, for example, which is part of the poem's long build-up to the start of the race itself:
The one hundred metres the race to be run
The alliteration of "race" and "run" calls readers' attention to the important event at hand. The quick repetition of that firm, rousing /r/ sound also makes the moment right before the race begins seem all the more exciting.
That excitement, in turn, makes it even more heartbreaking when one of the runners trips and falls, thereby ending his race and his dreams of victory. The speaker turns to alliteration again to highlight this moment:
But the youngest among them stumbled and staggered
The alliteration here is harsh and spiky, appropriate to an image of an athlete taking an unexpected tumble. The poem then captures the young man's frustration with a booming, punishing /d/ sound in line 18:
His dreams and his efforts dashed in the dirt
These /d/s are so insistent that they're almost violent, and they help convey a sense of crushing defeat.
Of course, the poem has a happy ending. The other athletes turn around and help their fallen comrade back up:
[They] lifted the lad to his feet.
Here, alliteration gives the line a little sonic lift. These lilting /l/ sounds are pleasant on the ear, making an uplifting moment sound more uplifting.