It's not always the hair cells. It's more the auditory nerves that are the problem starting out. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4227662/
As far as centralization, when it occurs and how to tell if it has, that's something I have often wondered about. Nobody seems to really know the answer.
You mean the auditory cortex, the auditory nerve is just the connection ("highway" of the signals transmitted by the hair cells).