This is actually a good question, because sound is subjective in its definition.
We all know that sound is essentially vibrations, but we classify only a sub set of these frequencies to be called sound. For humans, this is typically 20hz to 20,000hz. When the tree falls, it will definitely create vibrations in this frequency range, and if a human were there, we would hear it.
But what about the frequencies outside our range? If the tree were to make a 10hz or 50,000hz vibration, we would not hear it, and would say no sound was made, but ask a dog (if you could), and he heard the sounds we didnt. Does that mean the tree is not making a sound?
The purpose of the tree falling in a forest is to show us that what we call a finite, recordable sense is biased toward the human interpretation.
Another analogy I heard along the same lines;
If a jar of pudding is sitting on a table, does it have taste?
Of course it does have taste, but only when a human digests it does the taste become a reality, even though it was always there, but not measurable to us.
