First off silence as acoustic ecologist (yes, such a thing exists) Gordon Hempton defines it silence isn't the absence of all sounds, but as he puts it in Newsweek it is "the absence of all audible mechanical vibrations, leaving only the sounds of nature at her most natural. Silence is the presence of everything undisturbed." Second of all areas fitting that definition in the United States are declining: In 1983 in Washington state he found that there were 21 places with noise-free intervals of 15 minutes or more. Three years ago that had dropped to three. On average the most anyplace in the US is free from mechanical noise--Hempton cites air traffic as a prominent offender--is less than five minutes.
Living in New York City, I can tell you, that's probably less than five seconds. And you could certainly argue that continued separation of man from nature, mechanical from natural sounds, is part of the reasoning that got us into the situation of having fewer areas without mechanical noise in the first place.
But Hempton has a good point. There is something qualitatively different about being in places that are absent mechanical noise. Something that can draw you into a deeper place of contemplation.