There is a moving article in the NY Times Magazine this week about the work of scientist and acoustic ecologist Davyd Betchkal at the Denali National Park:
http://www.nytimes.com/2012/03/18/magazine/is-silence-going-extinct.html

Although I feel extremely fortunate that in my lifetime I have been able to experience real silence in the Dry Valleys of Antarctica, it’s alarming that someone would need to travel such distances to find it and frightening for the sake of all life that places of silence on Earth may soon disappear altogether.