Vibrations transmitted through an elastic solid or a liquid or gas, with frequencies in the approximate range of 20 to 20,000 hertz, capable of being detected by human organs of hearing.

So, the primary definition refers to being capable of being detected.... so the sound is exists upon action, not the reaction.

That may be a good enough definition for a general dictionary, but that business about 'approximately' and 'capable of being detected by human hearing' is not on for a scientific definition.

In any sizeable group of people, there will be a frequency that some can hear and some cannot. All well and good, that is still a 'sound' by the definition. Raise the frequency so that only one person can still hear it - it is still a sound. Now take every living person in the world as the group and there will still be a frequency that only one person can hear, but its still a sound. Now, let that person develop a bad cold, or die, and that frequency, which was previously a sound (because it was capable of being heard) is no longer capable of being heard by anyone. Is it no longer a sound?