I've watched Mustangs fly in every conceivable configuration. Gun ports. No gun ports. Taped gun ports. Two seats. One seat. Cavalier conversions. Desert. Grass. Concrete. Bs. Cs. Ds. Ks. Cuffed props. Uncuffed.
It's recording tech (noise reduction/protection?) more than any other factor that masks the hum. Films from the 70s actually do a better job than 99.9% of what we see today.
I've even seen multiple film of the same air show from identical locations (or from the other end of the field), like this one, and all of them sound different, with only one catching the hum.
It's the mic not the terrain. The sound wave propagates in all directions. Grass is not making any audible difference to the human ear.
Old Red Nose is making the hum at 0:07 but the mic is not good enough to fully capture it. On the second pass it doesn't catch it at all. But your ear hears it fine.
https://youtu.be/Ga53BtyIUt4?si=semJX9Yp1uKbGZnqHere again at 0:40, 1:51, 3:09, 3:50, and 3:58, the hum is being generated but the mic is grabbing the noise around it instead or it's being blanked by noise filtering.
https://youtu.be/1xrWUlDyUUs?si=ZsxgAicNCfQ0RNjo