As the air turns cooler in the fall, we experience many changes to the environment that signal the change in seasons, such as the way sound is carried through the air. Cool, dry air is denser than warm, humid air and makes a more efficient medium for sound waves to travel, so we hear sounds a little better.
Another reason is the frequent presence during fall of a temperature inversion. When the ground cools at night, the air near the ground is also cooled, creating a layer of air near the ground which is cooler than the air just above. This layer might be a hundred to a few hundred feet thick. The top of this layer is called the Boundary Layer, and it can bounce sound waves back to the ground, causing distant sirens and train whistles to be carried for great distances at night.