Hi Colin (@perspect) ,
I´d recommend to install a virtual sound device.
That way, you can select the virtual mic for those Zoom instances (rooms) which you don’t want to be heard in.
Also, as an added benefit, you can select the same virtual device as speaker for the Zoom instances which you don’t want to hear. Maybe you want to focus on one room and one room only for a moment?
I’m using multiple virtual sound devices and configurations but for your needs I can recommend Soundflower from Rogue Amoeba.
It’s not actively developed for a long time but hey, you only need a void device so it’ll be more than enough. Also, it’s free (as in beer and as in freedom).
You can read about it in their website then download and install from the developer’s Github page.
Best,
~ Fatih