Thanks for the reply. I should maybe have added that I run all my games with the basic premise that in the absence of immediate magical influence, the basic laws of physics apply. It makes running things a lot easier and fairer to the players, if they know that unless there is a mystical influence, the law of gravity will mean that things fall down, high up on mountains it is hard to breathe and the rotation of the planet means that night will follow day and vice versa. Things must make a certain amount of sense within the logic of a setting, otherwise players are not able to devise solutions based on these rules and are not able to distinguish between "this is just goofy" and "there is something actually manipulating things".
And this is one of these occasions where the explanation makes no sense at all. So I'm just trying to come up with something that makes more sense - I'd even take "there is a spell that prevents them from speaking above 200m underground, because...".
Take Vale of the Dead for a much better example: "The sound of the crumhorn damages the Hollow Rock." That's plausible enough and is something the players can work with. "The high pitch of the whiners' voices don't trigger the spores in the temple." Perfectly fine.
But the thick air comes with all sorts of logical problems, e. g. the dwelvers shouldn't be able to breathe before they lose the ability to speak, what effect would air of such density have on surface dwellers? And I don't have answers to these questions, which will inevitably come up during play, because there is no logic behind it.
I hope that made my troubles clearer
![Wink ;)](./images/smilies/icon_e_wink.gif)
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