I'm by no means experienced in this field, but wouldn't cochlear implants count to some extent? I know those sidestep the hearing loss rather than fixing it directly, but the end result is essentially the same. My mom has dealt with hearing loss since her early years of childhood, and peaking in total hearing loss about 15 years ago. Just a year or two later, she was included in a study and was able to get a cochlear implant for free. Not too long after, she was lucky enough to be offered a second one at no charge (the benefits of being a beta tester), and to this date, she's still being given new speech processors that constantly improve the quality of sound as well as adding new features. It honestly blows me away with what they've done already with hearing, and I won't be surprised if in 10-15 years, those implants surpass a normal human's capabilities, and people who still have perfectly working ears start getting them.