Often the best way to find information on a topic is to look for material published by a professional society or an organization.
American Speech-Language-Hearing Association
Judith Kuster's Net Connection for Communication Disorders and Sciences
National Institute on Deafness and Other Communication Disorders
National Parkinson Foundation
Useful Internet Resources: Meta links hearing and hearing disorders
Internet Institute for Speech and Hearing - Information about the science and technology of speech and hearing.