Interesting post! Really thought provoking.
Let us assume that the word "sound" means the actual vibrations in air which is not dependent of the process of hearing. Even if all human beings were wiped of the earth, the fact that an object hitting the ground will produce sound remains. I believe the laws of physics exist independently of the existence of human kind. Or what do you think?
Yes. Because, you know, science. Although it keeps the philosophers nice and quiet.
It's actually quite a presumptuous question, suggesting a person has to witness something for it to be classed as actually having happened.