Why the sounds outside the first can't be heard inside the forest.
Answers
I've always thought the point of this question was: if there's no way to establish whether there was a sound, then it ultimately doesn't matter. In fact, you could say that the definition of a sound is its detection, rather than the physical phenomenon. Of course, once we develop an instrument that can detect vibrations we couldn't otherwise hear, a previously non-existent sound comes into being.
This is a profound question with ethical connotations. For example, if we are unaware of somebody's suffering, does it exist? We can only be aware of it if they tell us or we infer it from our common humanity. But suppose vegetables suffered when we picked them, but had no way of telling us so. We might discover a vegetable nervous system, but we wouldn't know if the nerve-signals represented pain, information or something we have no conception of. The sensible conclusion would be to save your subscription to the Vegetable Liberation Front.