Gordon Hempton, who calls himself the Sound Tracker, is an "acoustic ecologist" who has traveled the world recording the sounds of nature, from birdsong and rainfall to babbling brooks and the rustling of leaves. But the noise we humans make is making it harder to find those quiet places - and, he says, it's having real consequences for wildlife as well. Bernie Krause, a musician and sound recordist, has become an audio anthropologist, documenting the sounds of nature. He also has noticed dramatic changes in some areas, such as in a Costa Rican rain forest. He helps correspondent Lee Cowan (and us) listen to the difference.