Because this poem is about the use of sound in poetry, sound is important in every line. But, stepping back for a moment, what does the poem sound like as a whole? To us it sounds like a fun game of echo. "The Sound must seem an Eccho to the Sense" (4) is the basic mantra of this poem, so it makes sense that reverberations are everywhere. Because each couplet is composed of a line quickly followed by its response, and because of the alliteration, meter, and rhyme, the response sounds similar to the line it follows – enough to sound like its echo.
(Psst. We talk a lot more about the echoing sound and alliteration in "Symbolism, Imagery, Wordplay," so be sure to check that out.)