It's weird, it's groovy, it's exciting, and it's all in one breath (not really, but close enough). Remember that without any punctuation, we can assume that we're not stopping the show at any point. Once we're in the audience's seat, our eyes and ears are fixed on the poet and we don't get any bathroom breaks.
Let's dig in.
The structure of the poem contributes quite a bit to the sound of the poem. We know it looks like one swinging thought so the sound has a similar swing to it as well, as the lines loop back and forth like a trapeze. And to top it all off, we have a nice big bag of magic-hat tricks (poetic devices) that stress both the excitement of the poem and the possible danger.
Let's start with assonance. We have lots of examples right from the beginning: "climb, rime, high, wire." Hear all those I sounds? Ah, but Ferlinghetti doesn't stop there. We also have an example of consonance in the repetition of the M sound in "climb" and "rime" (7-8). Not to mention the internal rhyming we see in these same words. Looks like Ferlinghetti may be showing off, and it makes sense, considering it's all a show in this poem.
We get more assonance later on in lines 11-12: "pace, way, day." That A has an elongated sound, meaning it takes some more energy to get the whole word out. And again, the sound here contributes to the long and challenging process of getting across that high wire and catching Beauty.
More consonance, too, in lines 13-15: "sleight, foot, tricks." Hear that short staccato T sound? All those short quick sounds roll right into each other, maybe mimicking the tricky nature of the poet's fancy, quick footwork.
Also we have lots of assonance (mistaking, thing / and, any) and consonance (what, it, not) in lines 16-18. The poem's structure seems to nestle these lines together, making them stand out, and the sound of the words does the exact same thing. These are important lines and Ferlinghetti seems to make that point through the combination of special structure and special sounds. After all, this is the poet's main responsibility: perceiving truth without mistaking "any thing" for what it "may not be." That T sound makes us annunciate "what, it, not" in a nice clear way. A truthful way maybe.
Later we get some alliteration in lines 19-21: "perforce perceive" and "taut truth." The repetition of these initial consonant sounds functions kind of like the T sound we saw in lines 16-18. It makes us pay attention and take notice of the words themselves and the ideas behind them. Again, it's the poet's job to "perceive taut truth," so these words are important. They've gotta stand out.
And we get even more alliteration in lines 22-25: "stance or step." We know these steps and stances are really just symbols for the various words and ideas a poet may work with in his advance toward Beauty. Just like the other examples we've seen so far, these words have the same initial consonant sound so they make us pay attention to their importance.
And last, but certainly not least, we have more alliteration in lines 32-24: "fair eternal form," and more assonance: "eternal […] empty […] existence." Of course Ferlinghetti would end his poem with a literary bang of devices. We're meant to see the true scope of Beauty's presence and the everlasting emptiness of existence, the combination of which makes the poet's work all the more challenging but also equally fulfilling if he's ever able to catch Beauty. So it all has a rather dramatic sound that's meant to pay respect to that elusive Beauty.