Interesting question and interesting responses. I don't have this problem yet, but probably will at some point. My hearing is deteriorating as a I age.

If it is an issue with the helmet, I wonder if an experiment is in order. This won't be popular but try riding without a helmet for one ride. Yes, there is risk but be extra careful and even more diligent than usual. Listen and note if there is less or more air noise. If it is the same, no point in looking at different helmets. Maybe it is something that just isn't fixable.

Though this won't help you with the person behind you trying to overtake you but there was a discussion on another thread about the Garmin Varia which might augment the hearing you do have on approaching vehicles.