One hand clapping...
The sound of a frame drum fading, and the profound silence that comes after in certain places.
The body-permeating sound/feel of an odaiko.
A violin or cello, if well-played. (If badly played it's one of the worst sounds...)
Voices in almost-perfect, but not too perfect, harmony.
The crunchy aliasing of bitcrushed, ring-modulated, distorted audio. (Synth and industrial music geek...)
Mourning doves, and the bird in our backyard that sings "germany germany germany dirt."
The white noise of a fan, at low levels. I miss it when it's gone.
My wife humming contentedly.