Yeah, I know about that. But that doesn't really help if the app only listens while foregrounded. (In that case the red bar does usually show up briefly before the app responds to the backgrounding event, but that's not very noticeable and basically a bug.)

Location Services is a model here: like sound recording it asks for permission first, like sound recording it flashes the menu bar if an app is using precise location in the background, but there's also a small icon that shows up in the menu bar whenever it's being used, even by the foreground app.