200 USD is eye watering? Sure you can get a better newer complete machine for less, but that’s more a case of used thinkpads being cheap rather than this being expensive IMO. Seems like a deal to me, given that you’re paying for parts (mobo + cpu, which also need sourcing), BGA soldering and shipping.
I have no issue with the stealth part given that it can only recognise audio which is already publicly available. If you don’t want people to know what you’re listening to, just don’t play it loudly in public.
This project also wouldn’t be possible if people were aware of it (it would just be gamed and certainly removed).