If I were to say, "Evolution is the idea that men are descended from chimpanzees," would you let me have my definition or would you say I was confused?

Usage. Dave interprets a sign from Jenny as referring to something, then he tries using the same sign to refer to the same thing, and if that usage of the sign is easily understood it tends to spread like that. The dictionary definition just records the common usages that have developed in the population.

For instance, how does the alien know what Takahiro means when he extends his index finger toward earth in this Japanese commercial? The alien just assumes it means he can find more chocolate bars on planet earth. If the alien gets to earth and finds more chocolate, he(?) is probably going to decide that his interpretation of the sign is at least somewhat reliable, and update for future interactions with humans.

This is the part I endorse.

"Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas."

It does not require the presence of a listener. Nor need it be in a certain range of frequencies. (That would just be a sound you cannot hear.)

What I am saying is, when Barry replies as he does, why don't we just say, "You are confused about what is and is not sound. Go ask the physicists, 'What is sound?' and then we can continue this conversation, or if you don't want to bother, you can take my word for it."

When physicists have a consensus view of a phenomenon, we shouldn't argue over definitions. We should use their definitions, provisionally, of course.

No one thinks it makes sense to argue over what is or is not an atom. I don't see why 'sound' should be in a different category.