If you lived on a planet with a very thick atmosphere, and you communicated verbally, what frequency range would be best? What would you eventually evolve to?
Would their voices need to be high pitched or deep bass? ultrasonic?
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You could have a read of this section of an article on outdoor sound propagation: Attenuation by atmospheric absorption (or a vaguely related physics SE question which brough that article to my attention). There are some fairly hairy formulae in there, and an important caveat "The formulas, are valid for a pressure under 2 atm, a temperature under 330 K and up to an altitude of 3 km" but one may extrapolate from that, especially as you're not asking for hard science.
TL;DR: the absorption coefficient is higher for a higher frequency and/or a higher pressure.
High frequency sounds will carry less well in higher pressure air. If you want to be able to shout as far, you'll need a deeper voice. I can't tell you which frequency range would be best without knowing about the atmospheric composition and pressure, and what metrics you'd use for "best" anyway.