If a squirrel is in the forrest, or a bird, or a microphone, or any other type of receptor, then there will be a sound. If there is not, then there is nothing to absorb the waves, therefore, no sound is made.
What if it's another tree, and the tree is shaped like a microphone?
Does that mean the "sound" does not exist? No waves would go out from the tree falling because there wasn't a squirrel near enough to hear it?