How much does the surrounding landscape affect how thunder sounds to us?
If located on a flat plain, will the boom have a less resonate sound than thunder heard in a valley? What about someplace with a decent covering of forest trees vs someplace with few or no trees? Rocky land vs sand or silty soil?
I think Acoustics 101 applies even when ramped up to that scale, but I suspect most variations in the sound of thunder are due to atmospheric conditions rather than the shape and constitution of the ground.
Anecdotally I can tell you that terrain does make a difference. I grew up living in a valley and now live in the mountains. The thunder around here really booms compared to what I was used to growing up, but that’s not a factual answer, just how it seems to me.
The “rolling” of rolling thunder is echoes. On a flat plane, you won’t get those. When I lived in Montana, surrounded by mountains, though, sometimes you’d get thunder that rolled for 30-40 seconds.
Not always. One end of a lightning bolt might be a lot closer to you than the other. The “rolling” is often due to the difference in time it takes the sound from different parts of the lightning to reach you. Clouds, atmospheric pressure, and precipitation also affect the way thunder sounds.
FWIW,
Hm. It’s hard to hear if it’s still rolling when the rain starts, and the thunder at the end might have been from a different event, with the lightning out of the camera frame, but that was still at least a good 27 seconds of roll. Clearly my explanation was incorrect (or at least incomplete), but I don’t know what the correct explanation would be.
The correct explanation is that almost everything affects sound. I think you have a better chance of finding two identical snowflakes than two identical thunder claps.
I’ve heard that thunder only happens when it’s raining.
I don’t think that was from a credentialed meteorologist, though.
Bah. Instant earworm. 
In previous discussions here the nature of rolling thunder was attributed to the length of the strike and the different paths taken. Speed of sound gets us close to 3 seconds per kilometer or 5 seconds a mile. So a 27 second roll would imply that the extend of the strike radial to the observer was about 9 kilometeres, which is not unreasonable.
There are significant temperature gradients associated with thunderstorms, so another aspect is reflection and refraction from and through gradients. I had always assumed that this is what modulates the thunder as the sound of the strike makes is way to the observer. If there is reflection from layers of air, reflection from the ground would become a factor, and indeed, one might expect that there would be some effect from the nature of ground cover. However absorption of very low frequencies is really difficult, and in the overall equation of the apparent sound, I’m not convinced ground adsorption would make much difference. Moist air alone is pretty efficient at dropping higher frequencies, and once the initial strike is heard the rolling sound is fairly consistent in timbre.
The type of lightning and the type of storm matters too. I’ve been through a couple of nighttime storms where there was a ton of lightning and some of it was very close, but it also was very muffled and crackly as opposed to loud and boomy. Also it went from scary frequent thunder and lightning to gone in just a matter of a few minutes. Whether that was a function of the thunder/lightning itself or the storm itself dissipating I don’t know.
Cloud-to-cloud lightning can travel insane distances and lead to storms having an almost constant rumble. That’s different than cloud-to-ground which is more what we usually think of. There’s also the occasional positive lightning strike which tends to be very powerful but doesn’t crackle and flicker so much, it just booms.
