Hmmmm. Regarding the tree falling when nobody hears it: we must merely apply simple logic.

First: if a tree falls -- even if we know it or not -- it simply occurs. Of course we don't know for we haven't been present to notice it, but that is irrelevant: what is relevant is the fact that we logically state "If it falls, it falls, regardless of our observations," and this statement is correct.

Second: what is sound? It is low/middle/high frequencies travelling through air and matter and does indeed occur regardless of our presence and our hearing perception. Short: a human entity is not on-site to detect the falling tree's sound, but nevertheless the sound did exist for the valid duration.