Sound waves that enter the ear ultimately trigger tiny waves in the cochlear fluid that are transduced into neural impulses by whose receptors are activated by the movement of this fluid.

Sound waves that enter the ear trigger tiny waves in the cochlear fluid. Movements in this fluid are transduced into neural impulses by the organ of Corti which can be found on the basilar membrane. It is considered the body's microphone. Its hair cells/ hair cell receptors are activated by the movement of this fluid. 

Ithink that its like 15 mph.