I suspect that #1 is the theory most likely to be true. I have personally discovered that a daily dose of sublingual vitamin B12 significantly improved my hearing. Sub-clinical B12 deficiency is at the root of several nerve problems, and could well be a contributing factor for dementia as well. Not the only factor, for sure, but if you have a hearing deficit, it's worth trying B12 supplementation. It's cheap and widely available, and non-toxic even in very large doses.