A 45 year old computer sales representative complains of tro…

A 45 year old computer sales representative complains of trouble understanding speech, especially in noisy situations. He complains of tinnitus in both ears. He contracted malaria some years ago and was treated with high doses of medication that he suspects was quinine based. The problem has existed for years, but it seems to be getting worse with passing time. Which of the following is the most likely cause of the problem?