The sound of an English country garden on a Sunday in summer.
This is a composite of several sounds all of which must be quiet or distant enough to not be a distraction but which in conjunction are glorious:
childrens’ laughter just far away enough to not be bothersome
bees buzzing from flower to flower
a propeller-driven light aircraft from a nearby aerodrome (ideally this would be a vintage plane with a Merlin engine, gently warbling in the distance)
the sound of leather on willow, and the occasional call and muted cheer, from a cricket match on the village green
the gentle burble of a stream
church bells, far enough away that their individual peels almost blend into each other
the clink of ice as someone pours a perfect gin and tonic.
When I was a kid this was how I knew spring and summer had come. One nested in the trees in our back yard and was very loud. I remember being scared of the sound for a few days before getting used to it.
I know it's way too broad and probably very generic of an answer, but music. Cannot live without it since I am pretty much constantly listening to it if I'm not watching a video.