For those that might be curious...
Sound travels roughly 750 mph, or approximately one mile every 5 seconds. The speed actually varies greatly with the temperature, but a good rule of thumb of 5 seconds per mile is a pretty good approximation.
The temperature of the atmosphere affects the thunder sound you hear as well as how far away you can hear it. Sound waves move faster in warm air than they do in cool air. Typically, the air temperature decreases with height. When this occurs, thunder will normally have an audible range up to 10 miles.
However, when the air temperature increases with height, called a temperature inversion, sound waves are refracted (bent back toward the earth) as they move due to their faster motion in the warmer air. Normally, only the direct sound of thunder is heard. But refraction can add some additional sound, effectively amplifying the thunder and making it sound louder.
This is more common in the winter as thunderstorms develop in the warm air above a cooler surface air mass. If the lightning in these "elevated thunderstorms" remains above the inversion, then most of the thunder sound also remains above the inversion. However, much of the sound waves from cloud-to-ground strikes remain below the inversion giving thunder a much louder impact. This also explains why you can see cloud-to-cloud lightning directly above you, but no thunder is heard.
