I'm not certain that I understand your argument, so I may have responded incorrectly. Let me know if you need any clarification.

On re-reading, I actually misunderstood your original point and my argument has nothing to do with your original point.

I would still want to point out a few things that may make what is going on clearer.

First, Brownian motion amplitude rises as temperature rises. So while the Brownian motion of temperatures typically found in the ear, or in the air near the ear, is small enough that the ear can't detect it, as you say, if you were to raise the temperature, the Brownian motion would be higher amplitude and would eventually rise to a point where it was detectable. This is a pretty academic point: the temperatures required to hear the brownian motion would harm the ear so in practical terms your statements are right enough.

If vibrations in the air cause the endolymph to have pressure waves in it which then cause cochlear hairs to move, it is still quite reasonable to describe that as air vibrations making cochlear hairs move. Introducing the endolymph is a clarification at best, not a correction.