There are no stupid questions only stupid opinions 8'D.
It is a question that is usually asked to ignite discussion. Plenty of people feel that there is no sound because no one is around to hear it while others feel you don't need to be around for sound to be made. It's like the whole is the planet a sphere or flat question? Yes, there are people who truly believe that the world is flat and you'll fall of the edge lol but it is still interesting to hear other people's constructive thoughts on such topics.
Trees are more polite when it's just animals around; they're super quiet. But when people start stomping around their forest, they tend to get quite grumpy. So making noise when falling is the only way to scare humans off.