Sound map of natural conditions of the United States by the National Park Service, estimating how pl

Sound map of natural conditions of the United States by the National Park Service, estimating how places would sound naturally, without human influence. 

The trend is higher sound levels in wetter areas with more vegetation. This is due to the sounds of wind blowing through vegetation, flowing water, and more animals (especially birds and frogs) vocalizing in more fertile locations.

You can look at higher res maps here and also see the sound map of existing condtions.