Mark wrote:a fact (action/sound/whatever) is still there (or has been there), even if it cannot be "proved". The fact in itself doesn't need to be proved, to exist.
If Schrödinger disagreed, but nobody was around to hear him, did he still have a point?
Or, to put it a little less facetiously, the work left in his wake is to the fact of his refutation as sylvan debris are to the fact of a tree's toppling in a forest. Forgive me for singling you out when in fact I respect what you're saying perfectly well; my tendency to philosophical ambivalence is easily overwhelmed by my enthusiasm for casual Elenchus. To whit, of all the rebuttals I might have chosen, indeed of all the positions I might have chosen to critique, these were simply the two most pleasingly paradoxical in their symbiosis.
In any case, what I intended to add has already been posited in its essentials; namely that I've always taken this to be a reminder of the semantic nature of much of what we take for granted as 'true' or real. We talk of sound as if it is a real world phenomenon(*) when in fact as a word it defines uniquely the sensation of such a phenomenon. I have always assumed, quite possibly incorrectly - I like some of the interpretations here - that the assertion that 'nobody is around to hear it' is an appropriately colloquial rendering of "there is no thing x such that x is capable of registering vibrations as 'sounds'". In effect I have tended to assume the question is something along the lines of 'is there any more to a sound (as distinct from vibration) than the fact of its being heard', to which I'm inclined to say 'no', but am always open to persuasion
(*)Here I lay aside questions of whether any such thing 'exists' given the neural nature of experience and mean simply to distinguish external/physical stimuli (eg an empty stomach) from internal/psychic perception (eg the sensation of hunger)