A falling tree in a forest would be heard if someone was there to hear it in the same sense that it would be seen if someone where there to see it. Now there are no trees, no sounds, no observers of them, real or imaginary. We are leaving all that behind to talk about sound in terms of our utilitarian definition of it, as a system of interactions, or rather as an interaction of systems that define both our use and our experience of sound as a phenomena.
What makes a sound?
Every sound, heard or not, is produced, or rather, represents (as "a wavelike disturbance in an elastic media" citation) an interaction between two or more systems. For it to be heard, as we shall see later, still more complexity is required. Firstly, however, here on earth a sound moves outward, wavelike, propagating from an event in all directions, unless obstructed. What was the event? It was a basic interaction between two physical systems like two snapping fingers, or a hammer and a piece of board (and again, a medium, as we shall see momentarily). A sound could be said to be the effect of this interaction, it is not always the desired effect either, as I unfortunately can attest to each morning when the construction begins outside my bedroom window. After all, those machines for sawing wood and shooting nails direct their energies towards the wood and nails used, and the sound produced is actually "radioactive waste" in a sense, or at least wasted radiation, similar to the way that heat produced by machinery and released into the atmosphere can represent lost energy in a physical system. Nevertheless, the cause of the sound is the same, be it a saw or a violin (and often there is little difference), two systems interact, energy propagates outward, and if we hear it we call it sound. In fact, when we organize it for amusement or communicative purposes it is called music or poetry, or some other things, but we must leave that for an entirely different discussion. The point is this: hammer and wood interact, air pressure is displaced between the two, a sound wave (which is a pressure wave in air and water) vibrates outward in all directions at a constant rate. This is sound production on earth, that is, inside of our atmosphere.
With the addition of the atmosphere we must increase our model by an order of complexity. Instead of two systems of interaction we now must include a third: the atmosphere. This environment, the medium through which our sound travels, is certainly of itself complex, but then upon consideration one realizes that so is our hammer, forged from complex mineral crystals melted at high heat and recast in an iron alloy, and the wood itself, which it strikes. Unfortunately those fundamental layers of complexity, those systems and their own fascinating molecular interactions will not be considered here. We must return instead to our earth-based common practice human definition of sound as audible radiation which depends on at least two interactive three-dimensional systems and a closed system of atmospheric pressure. This pressurized atmospheric room surrounding the earth provides "ground" necessary for the "figurative" changes in the pressure which allow it to become audible, and thus to fall within our definition of sound itself as an "audible" energy. Our definition, it turns out, is already recursive on one order, and also a bit self-centered: The pressurized atmosphere renders sound waves audible to us because it is the medium through which the waves that we can hear propagate.
Such circular thinking produces a crises of third order, for now it is clear than it is not enough to have two interacting systems interacting within an atmospheric system which forms the medium for their interaction but further that to hear any of this further complexity is required! So, then, our common practice definition of sound implies the presence of a listener. Earlier, it was determined neither to precede with an investigation of the medium (our atmosphere, in the example given) nor of the two interactive systems (the hammer and the piece of wood it strikes) in terms of their internal complexity. Still, a listener, a human listener, as implied by the common practice definition of sound as audible energy, is worth looking into. What systems are required for a listener to hear, and further, to make meaningful, the pressure data retrieved from the environmental medium as sound?
Depending on the organism in question these systems can be integrated or separate from one another. Most animal life, including humans, feature highly complex inter-connected systems of perception and interpretation (further reading reference here). Computers, to the extent one chooses to consider them to be organisms, often feature perception and interpretation systems discretely housed if not totally separated from one another, often dependent on user interface for systemic integration. The main idea, however, is that these systems, whether animal or computer, are frightfully complex, and find their greatest interdependence in that mysterious complex known as consciousness. Since we are discussing particularly human listeners, it is germane to explore how the human sense of awareness functions in the listening process.
Consciousness, as entity or idea, remains fascinating and strange. Despite the research into it being conducted in multifaceted ways, it's structure and function remain largely unknown. Thus, it will be approached in materialistic terms. Despite volumes of research into sound perception, it is clear that we do not yet understand how the perception of sound happens in the human brain in a clear way. I do not understand my own, and I certainly do not understand yours.
So then, what can be said about sound as a perceived phenomena?
For one thing, it is clearly not a static, momentary, individual, occurrence, but rather something in time. It is dependent on the three spacial dimensions but by it's nature also implies a continuity in another dimension. Sound exists in relation to time and has a shape in time, a history. This history, the perception of which could be said to be based on the memory of the perceiver of the sound, relates to the cause and the chain of integral effects within the sound, producing it's line of development in time. This is what musicians call the form of a piece or what electronic musicians refer to in terms of individual sounds as an envelope, the two dimensional line which describes the life of the sound from it's birth (attack), life (sustain), old age (decay), and finally death (release). It describes the perceived effects of the process of a sounds creation, existence, transformation, and conclusion, for any sound no matter what duration. Yet from this one sees that a "sound" is actually itself a complex, an aggregate of parts, of systems of interaction that through a person's choice have been made into one meta-system, that makes a whole that can be described by such an envelope.
What is required for such a choice to be made? What is required to take a selection of pulsations within an environmental medium and group them together as a concrete line of development and to claim that these together, through their interactions, produce one thing, this sound? The answer is memory. Yet the experience of this envelope, this line of sonic development as perceived by the listener says something more about the listener than simply that memory is required and the listener has one. To be able to interpret, much less acknowledge the existence of this time dimension, or to have a memory at all a special sense is required, a sense different in kind from the normal five of seeing, hearing, touching, smelling, and tasting. To experience the development of a sound, or of anything, one must ask what the sound is developing against, what the medium for this development is.
Similar to the momentary pressure displacement in the air required to produce the sound wave discussed in the first section, the medium against which the movement of the subsequent sound wave is gauged is that of time itself, as apprehended by an observer, with their own relative clock. If we had no memory, we would know no time, or possibly if there was no time there would be no memory, or stranger perhaps if there was no memory there would be no time (citation), and eggs before chickens before eggs into eternity.
Sound, in the widest physical sense requires no observer. A wavelike disturbance can and will continue to disturb it's elastic media in the absence of awareness of it by anyone. Otherwise, the universe would not exist, after all more than likely no one was around to observe that. Yet nevertheless sound still requires an interaction of at least two separate systems and a medium to exist. It is often difficult in practice to isolate these three systems, but nevertheless, they exist. Finally, a time dimension is implied, and perhaps the ubiquitous use of the word "wave" in definitions of sound implies this, as a wave itself is a periodicity, even if it is a modified periodicity. Sound requires two interactive systems, an elastic environment, and time. So to solve an old problem totally out of context: One hand cannot clap, especially if taken out of time and therefore produces no sound.