Over in "Form and Meter," we talked about the general structure of the poem that tends to look pretty orderly to us, even if it's not "meaningful." It's no surprise then that Ashbery uses a lot of poetic devices that also make the poem sound orderly too, while maintaining that smooth conversational rhythm we noticed earlier. There are quite a few sounds patterns to "Some Trees," whether we're talking about couplets, consonance, or alliteration.
Let's start with what we see first. We get regular end rhymes every two lines, whether a perfect rhyme (like "each" and "speech") or a slant rhyme ("are" and "there"). In both cases, Ashbery is suggesting that there's a definite sound pattern he's playing with. Sure, we may not have any meaningful answers here but the poem's regular rhyme scheme sounds seem to be making up for that.
As well, we get some examples of consonance in the third stanza: lines 9-10 repeat that R sound in "are" and "there." Can we venture to guess we'll hear more patterns in the next lines? We certainly do, with more consonance in the repetition of that N sound in "soon" (11) and "explain" (12). But we're not done yet. We have some alliteration too, with all those T and Th words in stanza 3: "to," tell," that," their," "there." All those similar beginning sounds force us to really pay attention to that stanza, since those words are practically throwing themselves at us.
All in all it seems Ashbery is spelling it out for us when it comes to the orderliness of his poem's sound patterns. He's kind of proving to us that sure things may seem and sound orderly, but if we dig deeper we may not find those meanings we're looking for. But hey, that's just the way the world works. And to quote Taylor from Kid Nation quite shamelessly, "deal with it!"