Cold air is denser than warm air and the molecules that make up the air are bouncing around less because they have less energy, hence the coldness. Sound travels as mechanical waves- the sound energy moves through the air by pushing and pulling the molecules back and forth as the sound wave travels. Cold, dense air allows for this mechanical energy to more efficiently travel because the molecules aren't bouncing around on their own as much so the sound energy goes farther before it gets completely dissipated. Leaves and snow may play a very small part in the process, but it's mostly the air density that makes sound travel farther in cold air.