--- but it's not quite what you might think.
Scientists have been dealing with the problem of natural tree falls (and the
sound they make--or don't make) for quite some time and have drawn some rather surprising conclusions.
If a tree falls and there is a person around the sound is easily recognized.
If a tree falls in the forest and there is nobody nearby, the sound that it makes is very different and often not recognized as the sound of a tree falling.
Either way, there is a sound.
Even though plants do not show any changes to the naked (or lensed)
eye, when a human is in their presence systemic biological changes have been discovered that have grave effects on plant life when a person is within 300 meters. This effect is called "human stress syndrome".
Apparently, when a tree is about to fall, if it senses a human nearby the biological stresses of human presence cause the cell walls in the plant to become brittle and it is the cell brittleness responsible for the familiar
sound we know as that of a tree fall. The cell brittleness also has significant effects to the quality of the lumber, making it much more suitable for use in construction (see below).
Through the miracle of recording tape, we are able to provide a sound recording of an actual tree falling without human presence.
Lumber from trees that fell without the human stress
factor are known to fail prematurely and have numerous imperfections. The photo at left is of a structure built (only three years ago) out of lumber that was found on the forest floor. 
On a visit to your local lumberyard you will note that
the coloring of the wood is relatively uniform and that the grain lines are clearly visible. The wood is also hard to the touch and dry. Some species of human-stressed trees create lumber with a greenish color (often referred to as "green lumber").
The non-human-stressed wood (naturally falling tree lumber) is almost always rather soft and wet. In many cases you will find splotches of various colors and sections of the wood that appear more like a sponge (one of the best sources of natural sponge is from fallen trees). If you try to build a structure out of this wood, you are in for a bad surprise when your building soon collapses.