Why does sound travel further at night?

Because of how temperature is layered in the air, not because the night is quieter — although that helps you notice it.

Sound bends toward regions where it travels more slowly, and it travels more slowly in colder air. During the day the ground is warmed by the sun and heats the air immediately above it, so the air is warmest at the bottom and cools with height. Sound leaving a source near the ground therefore bends upward, away from you, and a listener at a distance sits in a partial shadow.

At night the ground radiates its heat away and cools rapidly, chilling the air just above it while air higher up stays warmer. This is a temperature inversion, and it reverses the bending. Sound that would have escaped upward is refracted back down toward the ground, and can be refracted repeatedly between the inversion layer and the surface — effectively channelled along it. A train, a motorway or a distant conversation that is inaudible at midday can carry for kilometres before dawn.

Wind adds a directional effect. Wind speed increases with height, so sound travelling downwind is bent toward the ground and sound travelling upwind is bent away. This is why you hear a distant source clearly from one direction and barely at all from the opposite one, and it is often mistaken for the wind "carrying" the sound.

Lower background noise genuinely contributes as well. Traffic, machinery and human activity all fall away, lowering the level a distant sound has to exceed to be noticed.

The same physics explains why sound carries unusually well over cold water and across snow-covered ground, both of which chill the air directly above them.