Setting: This is a fanfiction set in the Naruto universe, some time after the current manga chapter. This means that they, theoretically, are about as technologically advanced as we are, with the exception of weapons, and their medics can heal reasonably minor injuries very quickly using chakra (just think of it as magic...). The main character of the fic (Sasuke) is now back in his home village, from which he was absent for... about four years, I think. He's now back to taking missions with his team.
Research: I googled 'hearing loss', 'permanent hearing loss', cochlear implant,' and 'hearing loss head trauma' among other, similar keywords that I can't remember now (sorry). I found out that permanent hearing loss is usually damage to the cochlea, and that a way to partially correct for it is with a CI, though in this case I don't want that to be an option, since the main character can't exactly carry around an obvious hearing device and still do his job.