Why does the thunder sound hollow?

Answer 1

Because it is lighting between clouds. Most of the sound originates

at the same distance from you. So the frequency of the sound is nearly the same. Therefor you can hear the harmonics created by the small difference in frequency.

Answer 2

Thunder can sound hollow because of the way sound waves are reflected and diffused in the atmosphere. When thunder rumbles, the sound waves can bounce off nearby objects, creating a hollow or echoing effect. Additionally, the structure of a thunderstorm cloud and the distance of the sound source can also contribute to the perceived hollowness of thunder.