@Pogsonator From the few games I tried, they settings in the game itself actually have mic volume sliders. Maybe you can try that and see if it helps?
Hey @Pogsonator, I would recommend you to leave feedback for this directly in the Stadia App (click on your avatar in the top right corner and then on "feedback") as this goes to the Stadia developers who can check that. Also if this issue is appearing also on your phone, it might worth to send the system-protocols there as well as this is an option you can send them.