Notice how the alliteration of s sounds in "sings," "hymns," at "heaven's" suggests a singing bird. There are four s sounds in the three words: "sings" begins and ends with an s and therefore the word "hymns," with its soft initial consonant, is similarly bracketed by s sounds. Shakespeare uses the word "sweet" in the very next line, an echo of the singing lark that has soared out of sight.
