Additional Study Information: The purpose of this research is to find out whether it is safe to use a virus to deliver a new gene to the inner ear of people with hearing loss. This is a kind of gene therapy which involves gene transfer. Once the genetic material and vector are surgically administered to you, it is not possible to remove these agents. The study is designed to evaluate the safety, tolerability, and the potential ability of CGF166 to improve hearing and balance (vestibular function). CGF166 is a weakened virus, made in a laboratory, that carries the Atonal gene. CGF166 is expected to help existing cells (supporting cells) in your inner ear become new, functional sensory hair cells.