Of course it does, something's existence does not depend on anyone's witnessing it. When you close your eyes, do the people around you become invisible simply because you're not looking at them? No, and it would be ridiculous to suggest that they did. Why should it be any different for hearing a tree fall? I know this question has been around for years, but it always seemed pretty silly to me.
Well what is the definition of sound? Is just the creating of sound, a sound? or does it have to be received? if the tree fell and the sound it made wasn't received, it could be that there wasn't a sound at all.