"The speed of sonorous vibrations through gases varies inversely as the square root of the weights of equal volumes of the gases, or, in other words, in the inverse ratio of the square roots of their densities." (Appleton's School Physics, page 377)

See Also

9.12 - Velocity of Sound and its Propagation Rate are Proportional
9.2 - Wave Velocity Propagation Questions
Compression Wave Velocity
Pseudo Velocity Shock Spectrum
Sound Faster than Light
Table 9.1 - Velocity of Sound in various Materials
Velocity
Velocity of Sound