I live in a place that is silent, for the most part. During the day you can hear birds, and the occasional sounds other animals make. Sometimes, I can hear my neighbors, but most often not. We are separated by stands of forest, and the leaves of trees are potent sound-dampeners. And, we have only a few neighbors for many miles around us.
At night I can hear crickets, and frogs, and other nocturnals, at least in the warm weather. But there have been plenty of colder nights I have stood outside with the dog, and the silence was so extreme that it was not hard at all to believe that there was no one else left in the world.
At extreme altitude, any fast moving object would have to be a jet. Wouldn't it?