I think you are mistaking how well sounds carry in still air for some other phenomenon. Balloons only go out when the winds are very, very light so that qualifies as still air.

I've camped on ridgetops above treeline and heard people talking and dogs barking 2000' below - very still night.

I've also camped in the valley below ridgetops where the wind howling sounded like jet engines up on the ridge. Had to wear earplugs to sleep that night.

I've tried using a rescue whistle to signal in snow and also in trees and found the useful distance quite short and even shorter if there is wind noise to ruin the signal-to-noise ratio.

So, my summary is, don't count on altitude having any helpful effect to signalling distance. Stillness helps, wind and sound absorbers in the path don't.

Unimogbert