Bernie Krause, a musician and naturalist, has for the last 40 years been leaving microphones in various habitats, making recordings of the wildlife. He's recently released a book, The Great Animal Orchestra, quoted in the Guardian:
"A great silence is spreading over the natural world even as the sound of man is becoming deafening," he writes in a new book, The Great Animal Orchestra. "Little by little the vast orchestra of life, the chorus of the natural world, is in the process of being quietened. There has been a massive decrease in the density and diversity of key vocal creatures, both large and small. The sense of desolation extends beyond mere silence.
"If you listen to a damaged soundscape … the community [of life] has been altered, and organisms have been destroyed, lost their habitat or been left to re-establish their places in the spectrum. As a result, some voices are gone entirely, while others aggressively compete to establish a new place in the increasingly disjointed chorus."
The article is fascinating, and sad. I'm not a huge fan of nature, personally, but this approach -- showing, tangibly, the holes in the soundscape that human intervention leaves -- makes it easy to relate to preservation activists. It's genuinely tragic to hear the world losing its songs.
I'd also like to add that this is yet another argument in favor of retreating, as a species, into massive supercities, and letting the rest of the world return to wilderness.