Sound is defined as :
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
The free dictionary

I think sound is being confused with communication by some here. Communication requires someone to receive the message. Sound just needs to be capable of being heard (not heard necessarily). If a deaf man stands was standing close enough to the fallen tree to feel all the vibrations, but didn't hear it, does it not make a sound?

The fact that we observe something does not make it so, either. We see stars in the night sky every night that burned out millions of years ago (according to scientists). So, some of the stars no longer exist even though we "see" them... Human observance or nonobservance is not required for something to "really" happen.