A falling tree creates vibrations in the air (as well as some damage on what it falls on). Vibrations in the carrier medium are not sound. Sound is when someone perceives those vibrations and therefore hears sound.
Original writeup preserved. December 7th, 2000 update:
My reply to piq: as the matter here is one of definition, I suggest reference to the word "sound". psykelus says that sound is "A vibration of a medium, a compression wave -- as interperted by the human ear.". Webster 1913 states, aside from all the other interesting meanings of the word "sound", that:
The peceived object occasioned by the impulse or vibration of a material substance affecting the ear; a sensation or perception of the mind received through the ear, and produced by the impulse or vibration of the air or other medium with which the ear is in contact; the effect of an impression made on the organs of hearing by an impulse or vibration of the air caused by a collision of bodies, or by other means; noise; report; as, the sound of a drum; the sound of the human voice; a horrid sound; a charming sound; a sharp, high, or shrill sound.
piq, if your stereo is playing away in the middle of the forest and there is nobody there to hear it, does that make any sound? How about if we take your stereo, start it out playing loudly and slowly reduce the volume until nobody can hear it anymore? Is it still making a sound? It's still creating vibrations in the air even if they are too weak for anyone in the room to hear them...
Sound is in the mind of the perceiver. I rest my case.
pfft has a point, but discussing that goes way over my head (for now, anyway).