The reason is that since sound travels faster in warm air than in cold air, the wave front becomes bent. The bending of a wave front between boundaries is called refraction. Refraction changes the direction of travel of a wave front. Consider for example that on calm, clear nights the air near the Earth's surface is cooler than the air further above the surface. Air at rooftop height above the surface might be 1 or 2 degrees warmer under such ideal conditions. Sound will travel faster in the higher, warmer air than it will in the lower, cooler air close to the surface.
A wave front will therefore become bent, of refracted, toward the ground on a cool night and you will be able to hear sounds from further away than on warm night (diagram 1). The opposite process occurs during the day as the Earth's surface becomes warm from sunlight (diagram 2). Wave fronts are refracted upward because a part of the wave front travels faster in the warmer air near the surface. Thus, sound does not seem to carry as far in the summer as it does in the winter. What is actually happening is that during the summer the wave fronts are refracted away from the ground before they travel very far.