You are not listening. Irrespective of pressure, sos is determined in a gas by the TEMPERATURE ONLY in a gas. IRRESPECTIVE OF PRESSURE. Pressure NOTHING to do with it. Density has nothing to do with it. Take a tank, pump out half the air, heat the gas back to room temp, and the speed of sound will be the same as to start with. PERIOD. The sound will be softer with a decrease in density, but it will not affect wave propagation velocity.

I fully realize that pressure drops with temperature decrease. This is not what I am talking about.