This is a tricky question which philosophers have been arguing over for years! I can't give a true answer because nobody really knows, but I'll walk you through the arguments.

When a tree falls and I see it, it makes a sound and I hear it. If it falls and nobody is there, we could assume that it makes a sound because it always makes a sound when we see it fall, but how do we know what it does when there is nobody there to witness it? This is a tricky question of "do things happen the same when we see it and when nobody sees it". But we can never answer that because we can never know unless we see it!

The second part is about "do things exist if nobody knows?". If a tree falls and I hear it, I am confirming that it exists. The tree is a thing because I hear it. But what if nobody hears it fall -- does it still exist if nobody can confirm that it does?

These are all tough questions that none knows the answer to, but philosophers love to debate it! After all, if there are no answers, we can keep debating and try to open our minds to different possibilities and different answers.