[E. Pluribus Unum]

"Since air pressure decreases with altitude and it takes less effort to compress less dense air, I speculate that it's easier to produce these sounds at high altitude."

I suspect that the lower the elevation, the easier survival is, therefore lower-elevation cultures don't have to work as hard at staying alive.

At higher elevations there isn't as much food simply growing on trees, so you have to work harder and interact and cooperate with your neighbors in more sophisticated ways, which requires a more sophisticated language.

[SunkenCiv]

The range of sounds made by Neandertal is now said to have been different; for (perhaps) most of the past 2 million years the ocean level has been hundreds of feet lower and the higher altitudes covered with glaciers.