Division and multiplication are reciprocal operations.

X / Y = Z -> Y * Z = X

Division by zero is undefined because multiplication by zero always equals zero. Reciprocity fails when you attempt to divide by zero. There is no value of Z that you can multiply by Y to get X if Y = 0.

As for falling trees, I've always believed that question's answer depends more on how you define the terms rather than physical reality.

"If a tree falls in the forest and no one is around to hear it, does it make a sound?"

From a physics perspective, the answer is a definite yes. Sound is a mechanical process. To suggest otherwise is to say that the Earth was completely silent before the first humans.

However, if the question is "If a tree falls in the forest and no one is around it hear it, does it make a _noise_?", then the answer is no.

Noise is a subjective thing depending on your perspective. To some people, a private plane flying overhead is noise. To me, it's a sweet sound, especially if it has a radial engine. To some people, listening to "rap music" (an oxymoron if there ever was one) is an enjoyable experience. To me, "rap music" is noise about as pleasant has having my teeth drilled.