Like an unbodied joy whose race is just begun. (15)
Now we're on to the happy side of things. The speaker imagines the skylark as a pure spirit of delight. We love the phrase "an unbodied joy." It's such a great way of putting the way he feels about this bird's song. Even this early in the poem, it's pretty clear to us that he's really having a kind of spiritual experience with this bird.
Thou art unseen, but yet I hear thy shrill delight, (20)
The speaker finds a bunch of different ways of describing the bird's happiness. In this case, he uses the words "shrill delight." He really wants us to focus on that sound, to think about the bird's song as an expression of pure joy. Slowly but surely, he's trying to pull us away from the idea of the skylark as a kind of average-looking, little brown bird. He wants us to see that this creature's song is really like an explosion of natural happiness.
Joyous, and clear, and fresh, thy music doth surpass. (60)
It's the pure, clean, fresh feeling of this bird's happy music that the speaker really wants us to understand. The point is not just that this sounds like a happy skylark; it's that the happiness is totally unmixed. It's not the slightest bit worn out, or muddy. That's part of what makes it so much better than human art, why it can "surpass" even our happiest songs.