It’s too soon to predict how much (if any) hearing loss can be reversed in humans, though very recent research on “Beethoven” mice is promising (the famous composer became deaf in his later years). The technique is called CRISPR-Cas, and even now is being used in China with human embroyos. To date there apparently have been no trials on living people, and it may be many years before such treatments become commonplace … assuming that they have been proven to be effective.
Gene therapy is relatively new and it may be proven to be useless, but if it is proven effective the implications for improved hearing are monumental. And the potential of improved hearing’s reducing dementia may also be monumental.
There has recently been similar progress in gene therapy for blindness although the treatment is very expensive. And in both hearing and seeing we suspect that it will take years for the processes to become reliable and cost-effective.