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.
A simple hang tag you pull out of the glove box and hang on the steering wheel that says: “By driving this vehicle you consent to audio, video and data recording” should do the trick.
$35 dashboard cams do audio.