There are camera systems that record audio. Put up a sign indicating you are on camera, and it’s “not secret”.
But most security systems like you mentioned don’t do audio. You could do it with a sign, but of course then you have this ugly sign in your very cool very expensive car. And in some states that won’t be good enough because the recording device is hidden, the nanny-cam industry ran into that problem, in some places you can’t sell stuff with hidden recorders even if it’s a selling feature. Surveillance device laws are very complicated and GM’s lawyers really should have done their homework on this a long time ago.