"Ars Poetica" spares no expense when it comes to blending the more classical conventions of poetry with the more modern. In the beginning it sounds mighty conventional with perfect couplets and poignant similes all about the art of poetry. Check out the first three stanzas for a refresher. But then we notice that the speaker is really doing something that's quite modern with all those paradoxes that make our heads spin: "A poem should be wordless." Does it sound as if the speaker is having a poetic identity crisis? Maybe. But then again, isn't all good poetry evidence of a perpetual identity crisis throughout mankind?
If you're nodding your head yes (or rolling your eyes no), then you've got a good handle on the sound of "Ars Poetica." It's lyrical and effortless while still managing to say something about the topic at hand. The speaker's airy and omniscient voice makes it all sound otherworldly, but he also reminds us of the need for poetry to "be" rather than "mean." And because the speaker avoids didactic allegories and "truths," we get the feeling that the poem as a whole sounds effortless and free.
The slant rhymes lend to the poem's effortless sound. Sure we've got plenty of perfect couplets, but the speaker balances the conventional stuff with the more experimental exercises in assonance ("releases" and "trees") and consonance ("wordless" and "birds"). So it's not all about sing-song rhymes and perfectly timed meters. It's more so about the speaker sounding as if he's playing with his own poem while also telling us some plain "truth" about the art of poetry. But even the truth doesn't sound so "truthful" in any textbook sense. It sounds open and free just like that "flight of birds" that lifts us out of the humdrum physical world of "words" and meanings.