Refraction of sound waves
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Sound waves would propagate rectarlinearly only if the speed of sound is constant in space and there is no wind. Since the sound speed depend on temperature which in particular varies with height, the sound speed is almost never constant in space. Moreover, the air is often moved by the wind that also carries the sound waves with different speeds. The additional speed of the sound by the wind also depends on height as the wind velocity increases with growing distance from the ground. 

In a medium with variing speed of sound the wave propagate no longer rectarlinearly. They are much more refracted, i.e. they propagate along curved lines. This also happens to the sound waves in the atmosphere. 

Refraction is often visualised by sound rays that descibe the direction of wave propagation like light rays.

 

 
Reduced audibility:
If the sound speed decreases with height, either because the temperature decreases with altitude and/or the sound propagates against the wind (upwind propagation), the sound waves are refracted upward. As a consequence, an acoustical shadow is forming into which sound energy cannot penetrate directly. Merely by diffraction and scattering sound energy can be shed into a shadow. However, it remains noticeably less noisy in a shadow zone than it would be expected from the distance form the source.
upward refraction (temperature lapse and/or upwind propagation)
Enhanced audibility:
If the sound speed increase with height, either because the temperature increases with altitude (inversion) and/or the sound propagates with the wind (downwind propagation), the sound waves are refracted downward towards the ground. At the ground surface the sound waves ate subject to reflection. The reflected sound is again refracted downward. A possible consequence is multiple reflection which is favourable to the sound propagation near the surface over large distances.
downward refraction (temperature inversion and/or downwind propagation)