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.