Just one small correction: the speed of sound in the Martian atmosphere differs from Earth, of course. On Mars it is much lower than on Earth. But the difference is not because of the low density or low pressure atmosphere on Mars. Instead it is due to the composition of the Martian atmosphere. Speed of sound is the product of gamma, and R (the gas constant), and temperature, (then take the square root of the product). Where gamma, the ratio of specific heats of the gas mixture, and R, the gas constant, both are functions of the composition of the gas mixture of the atmosphere. Nothing to do with density. The only variation in speed of sound in any atmosphere has to do with the gas temperature, as long as the gas mixture remains stable. Lower temperature produces lower speed of sound in any atmosphere.