What thou art we know not; What is most like thee?
In fact, the song of this bird is so amazing, so mysterious ("we know not") that the speaker can't find anything to compare it to.
Even after he has tried to explain the skylark using simile and metaphor, he still finds himself searching around for the right image: "What is most like thee?"
From rainbow clouds there flow not Drops so bright to see
So, these lines are like the first half of a non-simile—an anti-metaphor. Yeah, we just made those terms up. Don't worry, we'll explain: instead of saying that one thing is like another, the speaker says that one thing (the song of the bird) goes way beyond another thing (bright raindrops coming out of a rainbow cloud). Again, this bird's song is so great you can't even compare it to other things.
Doesn't this line kind of sound like something from My Little Pony? We think we might have had rainbow clouds on our binders in second grade. Okay, since Shelley is officially rolling over in his grave by now, we'll move on…
As from thy presence showers a rain of melody.
Here's the other half of the comparison that started in lines 32-33. The "rain of melody" (i.e., song) that comes down from the bird's "presence" is more beautiful and more wonderful than any actual rain, however bright and colorful and magically delicious it might be.
Of course this is a metaphor too, right? Melody isn't really a rain. Shelley puts our senses in a blender in this poem. We see music, we hear colors. The fancy term for that is synesthesia. In this case, it's all about making us feel excited, confused and amazed by this bird's song.