"If a tree falls in the forest and no one is around to hear it fall, does it make a sound?"
Despite the seeming profundity of this question, and despite its use in philosophical discussions and philosophy textbooks, it is really quite silly and is in fact obsolete as a philosophical issue in the 21st century. What is wrong with the question is that it frames the issue in false terms. It makes it sound as if the tree is conscious (in the sense that humans are conscious) and is able to detect the presence of humans; the tree decides whether to make a sound or not; and it follows the rule: When there is a someone around, make a sound when you fall; when there isn't anyone around, don't make a sound. The question makes the phenomenon of sound into something like the equivalent of an arboreal fart that the tree holds in or lets out at will depending on the social circumstances: Fart when you are alone or with your wife, but don't fart in the presence of company. Expressed in this way, the question can be seen for the nonsense that it really is.
To get out of this corner, some "philosophers" resort to the "semantics" expedient and say that it all depends on how we define the term "sound." If we define sound as the effect of certain fluctuations of air pressure on the tympanum of the human ear, which sends signals to the brain where they are interpreted in a certain way, then the answer is "No." However, a little reflection will show that the question is still absurd. If sound in defined in this way, then it is not something that the tree "makes" but something that is created in the brain. It is impossible to talk of a tree making a sound, if by "sound" we are referring to the effect of certain physical phenomena on our sense of hearing. If by definition sound consists in the act of reception, then we make the sound, not the tree.
Of course, from a scientific point of view, it is nonsensical to define sound as the act of reception. Science has long know that there are sound frequencies beyond the limits of the human capacity to receive. One might ask: If someone blows a dog whistle and there are no dogs around to hear it, does it make a sound? This, of course, would be a truly nonsensical question in science. Similarly, one might ask whether bats make sounds, since humans do not hear them.
Today we understand the "mechanics" of sound fairly well and there is little doubt as to its nature and how it is produced. We know that sound is not something that happens in our ears or our brains: it is merely the perturbation of air. Sometimes these perturbations have no effect on our ears, or if they do, our brain is unable to process the effect in any meaningful way. Thus, the phenomenon is in the air, not in our ears. Let us imagine that there is a large crowd of people gathered in the forest, and that there is a given tree that the people are watching. Let us further imagine that this tree is enclosed in a giant glass dome from which all the air has been sucked out to create a perfect vacuum (and let us also imagine that trees, or this tree in particular, can survive in a vacuum). Now let us imagine that we somehow contrive to make the tree fall. There are plenty of people around to hear the sound of the tree falling. But does it make a sound? Of course not! This "thought experiment" should be sufficient to debunk forever any notion that sound is a human construct and requires the presence of humans (or living creatures). That idea comes from the heady anthropocentric philosophies of the Age of Reason, when man became the measure of all things and everything in the universe was understood from the reference point of the only creature capable of "reason."
That intoxicating anthropocentrism was nowhere more eloquently expressed than in the following lines written by the English poet Thomas Gray (1716-1771):
Full many a gem of purest ray serene
The dark unfathomed caves of ocean bear:
Full many a flower is born to blush unseen,
And waste its sweetness on the desert air.
—"Elegy Written in a Country Churchyard" (1750)
The idea here is that if humans are not present to witness and appreciate the beauty of nature, it is wasted. This, of course, is sheer arrogance, since man has been around for a mere fraction of the time that gems and flowers—and trees falling in the forest—have been around. It is time for philosophy to shed the obsolete thought patterns of the past and catch up to science. Philosophy today needs to direct its energies to more fruitful lines of inquiry. One way to start is to frame the issue in more meaningful terms: How does the human perception of sound shape our understanding of the world around us and our place in the universe?