If you're all about the assonance and alliteration, well, so was Wilbur. And this poem is chock full of both of those poetic devices.
The first line, for example, has the internal rhyme of "eyes" and "cry," followed closely by the alliteration in line 2: "spirited," "sleep," and "soul" (plus "astounded," if you want to throw another s-sound in the mix). Keep reading and you're bound to find more repeated sounds.
They have a way of linking the poem together, of pushing you through from line to line, driven on by familiar sounds. But they also help Wilbur make meaning, as in line 10, which has the alliteration of "feeling, filling." The alliteration helps fill up the line with sound, mimicking the meaning of the line. It's a little dose of sing-songy joy, just like the joy these angels are feeling as they breathe in the breeze.