Baloney. Sound is energy. Energy is given off as a result of the tree falling. Some of that energy is in the form of sound waves. They do not need a human or hearing witness to "exist."
I agree hocndoc, the sound waves do exist, but does the sound? Certainly a deaf person can stand there with an instrument and measure the changes in air pressure that occur when the tree falls down. But the deaf person did not hear a thing.
The point is this: yes, things exist apart from our human perceptions. But can things exist apart from the perceptions of any conscious rational being, human or something else?
Can you perceive a gravity wave from the origin of the universe? Can we measure the background radiation from the Big Bang? ... But until we could measure it, was it not there?
Sorry, but you don't need to consult a philosopher. What you need is a lexicographer.
Energy exists at those wave lengths. They exist in a room full of deaf people.
Now then, as to whether anything could exist without some sort of conscious being: I am a person of faith. I don't believe that anything could exist without God. But, that's faith, and except for the orderliness and the fact of existence, I don't have empirical evidence for God, yet. I'll have to get back to you when I find it.