Well, a sound is a form of energy that travels through the air as a wave caused by the vibration of molecules. Humans can hear this sound if the frequency of the wave is (I believe) between 41 and 20,000. A dog, however, can hear outside this range, so we cannot limit a "sound" to be something a human can hear.
So, a sound is therefore a form of energy caused the vibration of molecules. When a tree falls, the molecules in the tree, the ground, and the air vibrate, so it makes a sound - even if someone is not around to hear it.
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