To clarify what I believe is the question: Why can't solipsist hear brownian motion?
The question is pretty good; Brown Noise derives its name from brownian motion, or rather the discoverer of such, as it is the frequency (or set of frequency) that brownian motion produces.
I'd -guess- the answer is that the motion all cancels out on the average, approximately, and the remaining statistical noise isn't energetic enough to be perceived.
This thread is for asking any questions that might seem obvious, tangential, silly or what-have-you. Don't be shy, everyone has holes in their knowledge, though the fewer and the smaller we can make them, the better.
Please be respectful of other people's admitting ignorance and don't mock them for it, as they're doing a noble thing.