In heat lightning you see the flash of light from the lightning, but you don’t hear the thunder the lightning produces. All lightning produces thunder when the bolt heats the air around it. This rapid heating causes the air to quickly expand and generate a sound wave known as thunder, which travels through the atmosphere. The temperature and wind structure of the atmosphere can lead to an acoustic shadow zone where you can see the source of the sound (the lightning bolt) but not hear the noise (the thunder). Topographical obstructions can also lead to an acoustic shadow.
Sound can interact with objects in multiple ways. As sound waves move through the atmosphere, they are absorbed by molecules in the air. The waves can bend around objects, such as trees and leaves, or be absorbed by those objects. This will muffle the sound. Rapid changes in wind speed or wind direction can influence the direction of a sound wave, which can also affect whether we hear it.
Sound waves travel at a speed of about 12 miles per minute. The speed is a function of the temperature of the air. In general, sound travels faster in warm air and more slowly in cold air. Since the temperature of the atmosphere changes from point to point, so does the speed of sound.
As sound waves travel through the open air, they can change the direction in which they are traveling. In the lower atmosphere, the temperature of the air generally is cooler with increasing height above the ground. Cooler air over warmer air causes the sound wave to bend upward away from the ground. In this condition, you may see the lightning but not hear the thunder.