Sound in air
For air, g = 1.4, R = 8.3 Joules/mole K, T is the temperature in Kelvin, and M (a mixture of N2 and O2) is .029 kg/mole.
Thus at room temperature (75oF=24oC = 297 K), v = [1.4 * 8.3 J/mole K * 297 K / .029 kg/mole]1/2 = 345 m/s = 770 mph.
At higher altitudes we have lower temperatures and hence lower speeds.