Situation........Your argument.........My counter
If not observed, and no sound is made, the tree never fell. - agreed
If not observed, and sound is made, the tree fell. - the tree falling can not be proven, the sound could have come from something else
If observed, and sound is made, the tree fell. - agreed
This is the essence of the question. To know the tree fell it would have to be observed. This contradicts the question when it says no one was around to hear it.
edit - bonanova said it better