The poem follows a dog, though we only slowly realize it. She hears something, turns her head, and her ears and tail go up. The brown and white fur and the barking give her away, and the "monkeys" calling her back are the humans, whose sensible commands sound like nonsense to her.
The speaker stays close to her excitement. The sound moves across the grass, the wind-bent oaks and the white and rose-colored house, and it's described in rich, shimmering colors: "aged paper" and "scintillating emerald." It's "Not music, something more. / Not silence, less." The repeated "A sound!" makes the thrill feel urgent and fresh each time.
Then comes the quiet ending. She races toward it, and there's nothing: "No sound. / It's gone. / Oh well." The mood moves from wonder and eager curiosity to a shrug, which is gently funny. The poem suggests that the chase itself mattered, and that the dog takes disappointment lightly.