Totally passive aggressive bs, in my opinion. It may be that depression and/or chemical changes in his body are contributing, but those are NOT an excuse for this type of behavior. It is common for men to have testosterone levels drop in their 40s or later and it does leave them with lowered energy, maybe contributes to depression, etc... but it still is NOT an excuse for rudeness or p-a behavior.
Why not is NOT a question I would ask. Seems rather like asking your two yr old if he wants to go to bed. (I want to SMACK parents who do this and then are upset when the kid won't go to bed - WTF are they expecting??? Of course the kid doesn't want to go to bed, duh!) If you respond iwth 'why not" you are going to get all sorts of reasons why not - most of them being I don't want to do it and want to upset you because I am too childish to speak to you like the adult I am supposed to be about what is bothering me.
I would use "because I said so" or "because you need to do this today". If THAT gets a "why" then it would be because you want me to do things that benefit you - like pay bills, make meals, do the shopping, buy things you want/need/like, etc..." I would also be very open that I won't tolerate it anymore and if it continues life might get really really ugly for a while until he chooses to behave like the adult he is suppsoed to be.
It is one thing to say "what?" or "excuse me?" if you didn't hear. LOTS of times if someone is doing something and you speak they may miss some or all of what you say because they were paying attention to what they were doing. In that case, it is reasonable to ask to have the request repeated. This is the reason I used to say "chocolate chip cookies" or "chocolate pudding" or some other favored dessert item when I wanted one of my kids, esp Wiz, to do something. If I spoke, even said his name and that I wanted to talk to him, I did NOT get his attention. But any mention of something with chocolate could get his attention from down the block at a friend's house. So I used it.
I have a friend who's husband used to be bad about not really paying any attention when she told him something. While chocolate pudding didn't work on him, if it was important she took her shirt off. THAT worked. She also did it when he was gritching about something stupid - stopped his arguing cold because all he could focus on what NOT the words coming out of his mouth.
I do htink that even if there are hearing or attention issues, your husband is also being very passive aggressive. I hope at some point he can choose to learn a better way to communicate with you.