I have no answer but propose a simple and easy-to-execute experiment to find the answer, since answers given are slightly contradictory or unsure of themselves.
Take a sound generator (such as a boombox or a simple bicycle bell and bicycle horn even) to a place where there is clear reach to a distant wall, at least 2-300 metres away. Sound both at the same time and listen to the echoes coming back. If there is a material difference between speed of propagation between high- and low-frequence sounds, the "faster" sound you should hear slightly before the "slower" sound.
By the way, I don't buy the "longer to generate" theory, typically applied to thunder and how it sounds to the human some distance away. If you are NEAR a lighting striking, you will NOT hear a high frequency first, and then a few seconds later a low frequency sound. They don't get generated later. They both get generated instantaneously. I mean, why would they get different generation time? The lightning does not last seconds, whereas the deep sounds last up to ten seconds some distance away from the lightning to the human ear.
What I think happens is that the deep sounds have a larger propensity to be echoed by the medium itself that carries it. Much like a Pringer-Hoffsteiner effect, the wave itself creates and behaves in an elastic manner, and it does not behave only in a straight-line propagation manner. It pushes the air forward, which will repush some of the sound backward, and which gets pushed forward again. Hence the effect of a longer, much longer rumble of thunder by a lightning, while the high frequency sounds do not have much energy, therefore they have a negligible Pringer-Hoffsteiner effect.
This explanation of lightning does not explain which travels faster, the lower or the higher frequency sound. They may travel at the same speed, but as the deep sounds are elongated, their intensity may be spread, and it may reach the listener at the same time as the high-pitched sound, the high-pitched comes loud as it hasn't lost amplitude due to dampening; and the low-frequency may initially arrive at the same time as the high, but due to dampening and its energy dispersed into a longer time interval, it may be muffled or just not attended to by the human ear.