The hair cells are arranged in layers with the outer hair cells being closer to the outside, sort of in front of the inner hair cells. The latest theories hold that the outer hair cells flex or stiffen to protect the inner hair cells from loud sounds, or to attenuate the louder sounds. You can think it of a gain adjustment like you have on your television set. You adjust the overall sound level so that you can hear the TV show without being blasted by the loud sounds. Sometimes you have to turn the set up or down to accommodate the different show segments.
Commercials often have to be turned down, for example.
Well, the hair cells in a normal ear do the same thing. The inner hair cells tell the outer hair cells when the sound is too loud. This causes the outer hair cells to stiffen and reduce the amount of sound vibrations getting to the inner hair cells, thus lowering the volume. In some kinds of hearing problems, one of the first things to go is the ability of the outer hair cells to control the amplitude of the sound. You sort of loose your automatic volume control.