The deafness is related to pattern, splash white (not bald faces). It has to do w/ the lack pigment cells inside the ear, you can't tell by looking if they are pigmented or not, and yes ALL horses that are lacking that pigment are deaf.
I have a deaf great dane, and it is related to her pattern. Not all white great danes are deaf, some merlequin great danes are deaf, if they have pigment inside their ear they can hear, if not they are deaf (less white on their head gives a better chance they can hear, but is not a guarantee). (in danes (and dogs that come in merle) it goes along w/ the merle gene). You could call it genetic in the since that both her parents carried one merle gene, but it's only an issue when both parents pass them. I could breed my dog and as long as I breed her to a black or mantle, or some other non merle dog there wouldn't be any issues w/ her passing on her deafness.
Sorry, hard to explain, I hope that makes since!