It's an interesting philosophical question.
One might immediately answer, "Yes!" without much thinking.
To answer the question, we should define what sound is. Sound is mechanical vibration transmitted by an object. Hence, when the tree falls, it will certainly make a huge vibration.
But does it make a sound?
Every atoms in our body vibrate, but we can't hear it. In order for us to call something a sound, it has to able to be heard as well. If the tree falls and no one hears it, then the sound doesn't exist!
So far, the explanation is quite technical. Any physicist can answer it, how does this question philosophical?
Above all, I think the question tackles not just merely the sound, but existence. How can we say something exist? If we were to modify the question to be:
If Jengtu had lived in Beijing in 980 C.E but there is no record of him and no one knows him now, does Jengtu exist?
How can he exist if there is no trace of him? The only reason we know Jesus, Michelangelo, and all those dead people exist because we have records and traces of them. If we don't have any trace, then certainly Jengtu doesn't exist. Jengtu might have existed, but right now he does not exist.
It seems that we have reached the end of our conclusion.
A philosopher should not so easily satisfy.
A philosopher should follow both his reasoning and intuition and gut.
Something is not quite right in the question:
If a tree falls in a forest and no one is around to hear it, does it make a sound?
How can someone ask such question?
In order for someone to ask this question, somebody must have known a tree is falling. If we know a tree is falling, that means there is someone around the forest, isn't it?
How can you know a tree is falling without being there?
Even though you say they are just all inside you mind, you still is inside your mind in order for you to know.
Hence, the question is invalid.
Does the conclusion make sense?
One day, someone walk around the forest and saw a fallen tree. He was not present when the tree was falling, yet he knows that the tree fell.
Then this statement contradicts my previous argument leading to me concluding the invalidity of the question.
In the end, I still don't have an answer to the question.
What do I mean?
The depth of a philosophical questions lie in the limitless of arguments one might come up with to support one's view. It may seem I already have the answer to some, but I have no doubt one day, someone or even myself may come up with a counter argument to disagree with myself.
There really is no answer to the questions like:
What is life?
What is my purpose?
Does God exist?
The best we can do is to believe and to have faith.
"If you do, fair is the prize, and great is the hope, and it is in that hope I can go happily to my death."
Ironically, Plato's statement is not a truth.
We can only believe him.