Sigh....you know, every time this question comes up, everybody seems to catastrophically miss the point. The issue being investigated here is whether events not significant to the universe at large are significant at all if they are not perceived, or if they can even be considered events, and the reasonable answer to that question is, "Sure, why not?"
We can talk about physics and pressure waves all day and address the question with science, but at the point that you're doing science on the problem you've already perceived the event in your mind.
The proper answer is that the question itself is invalid. At the point that you propose that a tree has fallen, YOU HAVE ALREADY IMPLIED THE PERCEPTION OF THE TREE FALLING. I cannot conceive of a tree falling without making sound in any normal environment (no vacuum allowed), therefore if I understand that a tree is fallen, I understand as a matter of course that it must have made sound on the way down. While it is true that you could say that if I never knew the tree fell my universe, as filtered by my perceptions, would never register the existence of any sound at all, thus allowing for the tree to attain the state of "fallen" without registering the perception of "sound," as soon as I am informed that the tree has fallen, presuming that I am not a blithering idiot and have a basic understanding of cause and effect, I will immediately presume that the sound happened.
The question, then, is self-defeating.