The sound of massive double overheads crashing into the beach - a symphony of nature's power - juxtaposed to your sitting in a chair, without a worry in the world, sipping on a beer as the light warm wind blows by. Sort of like nature's own contrapuntal sound track.
A good low frequency rumble in an otherwise quiet environment, such as a late night flight where most people in the airplane are asleep. So serene, despite the 200 people around you. Then again, I just find the inside of an airplane in general quiet soothing because of it's unique acoustics.
Sitting on the bow of a sailboat while sailing in a beam reach. The combination of the hull crashing through the waves and the wind whipping through the sails (and NOT LUFFING of course!!!) is probably the most soothing sound I can think of.
The clinks, clanks, and whirrs of a nice steam room are a wonderfully relaxing sound.
The sound of my 6.0L V8 as I shift down into 3rd to jump on the highway (music turned off of course! My car makes it's own music! )
The whirring of a nice, big server room, listening to all the computers do way more than you could ever imagine.