There are many types of dementia: Alzheimers disease, vascular dementia, dementia with Lewy bodies, frontotemporal dementia. Thus, there are many neurodegenerative mechanistic pathways that end up with 'dementia'. There are also many types of hearing loss. But most dementia types and most hearing loss are age related. I would say that both dementia and hearing loss in general are symptoms of a neurodegeneration (an ageing brain). I do not at all buy the hypothesis that you can prevent dementia by preventing hearing loss.