This is actually a philosophical question taken from epistemology. It can be used to address the opposition between rationalism (knowledge from reason) and empiricism (knowledge from senses). According to the rationalist, we know the fallen tree will make a sound by just thinking about it; the rationalist says the objective reality exists independent of our senses (e.g. the apple is still in front of you even if you're blind), and thus it is logical possible that it did make a sound. According to the empiricist, we never know until an observer receives the sound through one of his or her five senses; the empiricist says reasons alone only tell us definition and nothing about the real world (e.g. by definition, all bachelors are unmarried, but so what?). To me, Plato was wrong that "all knowledge comes from reason," since we do have to use our senses to know something as true or false; we have to use both reason and sensory experience to gain knowledge. Pure rationalism is unrealistic and illusionary, while pure empiricism is subjective and deceiving.
Of course, with the clash between rationalism and empiricism, the question still remains unanswered, "Do we know anything?" And that just goes back to the original question in this thread.