Ah but when the tree falls, it creates that disturbace of the air as you stated, much like when an object is thrown into a pool. The air, being fluid in nature, react just as the water did, by transmitting that disruption's energy in the form of waves, at a particular frequency. And yet, we don't hear the waves traveling though the water, except those that are within our hearing range. We call water waves silent. And though we may feel very low frequency air waves, we don't call that sound, as we can't hear them.
I submit that the definition of sound is that frequency of fluid that strikes the proper sensory organs to be interpreted by the brain as sound.
Therefore, if there is nothing to hear the sound, then it is simply an energy wave propagated through a medium, not sound. In other words, it isn't sound until it is perceived as sound, be it a bug, or a person.
All discussion about this topic is welcomed. To me, this could be an interesting discussion.
Seeeeeya; Chief Longwind of the North
But if the person present is deaf? The sound is still there, just not perceived as sound. Add in a second person who is not deaf.
Sound can be perceived as an air pressure wave...