I never wrote part 2 of this blog post, but there's a $300 open source hearing aid that can do way more than most hearing aids: https://shapr.github.io/posts/2019-08-03-tympan.html

Here's the organization building this thing: https://shop.tympan.org/

I'm kinda excited about the four microphone board, where you can subtract the ambient noise behind the listener from the noise in the direction they're facing.