Louise Glück is a sly poet. And her use of sound is just as subtle as her manipulation of meaning. When you conduct a preliminary sound check of "The Wild Iris," you won't find a lot of honks of the horn. There's no end-rhyme, for example.
Still, if you take a closer look, you'll discover several incredibly cool uses of sound to enhance meaning.
Check out the repetition of the word "returns" in line 19: the word shows up at the beginning of the line and then it returns (get it? get it?) at the end of the line. And consider the phrase "You who do" in line 16: all three words rhyme. Shmoop thinks that the rhyming words emphasize the urgency of the speaker's desire to communicate (yoo-hoo!) and create an echo effect that spookily reverberates from the "other world."
The poet also uses consonance (repetition of identical or similar consonants) to emphasize connections among key concepts. Consider, for example, how the repeated v sound links the ideas of "oblivion" and "voice" in lines 19 and 20. In the final line of the poem, the sh sound in "shadows," joined to the zh sound in "azure," alerts us to a mysterious transformation of darkness into blue light.