The basic formula for the speed of sound in a gas $c=\sqrt{\left(\frac{\partial p}{\partial \rho}\right)_s}$ assumes that it is uniform, and thus that sound waves travel through the gas at one speed. Suppose a gas is composed of multiple components with very different masses (e.g. a 50/50 mix of hydrogen and carbon dioxide).
Is there ever any case where it is more accurate to model the transmission of sound through this gas as happening at two distinct speeds instead of one?
Specifically if the gas were confined to a thin two-dimensional film between two solid boundaries, and it is thinner than the mean free path in the gas, then sound waves transmitted by the gas molecules will be better approximated as happening at two speeds than one. If that's the case, you could do lots of fun things with thin-film style interference of the sound waves.
That said, when dimensions are that small, even talking about "sound waves" is dubious. That's why I'm curious about whether this idea works, especially whether experiments like this have been tried.
