The scenery was very pretty, he thought, and there were new things to look at, but it was all a green and blue blur and he was used to the gray and black of the city, the sounds of the city. Traffic, people talking, sounds all the time—the hum and whine of the city.
Here, at first, it was silent, or he thought it was silent, but when he started to listen, really listen, he heard thousands of things. Hisses and blurks, small sounds, birds singing, hum of insects, splashes from the fish jumping—there was great noise here, but a noise he did not know, and the colors were new to him, and the colors and noise mixed in his mind to make a green-blue blur that he could hear, hear as a hissing pulse-sound and he was still tired. (4.52-53)
At this point in the book (just after the plane crash), Brian is bewildered by the natural world around him. It's totally different from anything he's known before in the city, and it's totally overwhelming to him. What he first hears as a silence turns out to be full of sound, full of life—right away he has to start adjusting his understanding to his new reality.