They provide some, my Schuberth is waaay quieter than my older ones but nowhere near anything useful at any speed. Earplugs for every ride.

No, helmets don't provide protection against hearing damage, or at least not enough protection.

Absolutely not, unless you are going very slow (like bicycle speed). The wind noise even with a good helmet is deafening and well above OSHA limits for workplaces. I would never ride without earplugs if my trip was over 10 minutes or above 35 mph. I did it a few times when I first started and I would get off the bike after 30-45 minutes and It felt like I had been to a rock concert. With earplugs I can go as long as my back allows and I'm not fatigued afterward.

Helmets provide little noise reduction. I usually wear earplugs under my helmets.

No, in fact that's where all the noise is coming from. The buffeting of the wind against the helmet.

After solving for fall protection, hearing is the next thing on a motorcycle.

Helmets don't protect against wind noise, can actually make it worse if you're using one that has a sun visor.

Every time I'm going "flat out" (not inner city) I wear ear plugs and seriously regret it if I forget them.

None at all, wear earplugs.