> It's simply impossible for FB to record audio.

You should check your sources again - that statement is false.... You only have to look at the release notes from the dev preview on Android P today to see that was possible for any background app to access the camera and microphone at any time:

https://developer.android.com/preview/behavior-changes.html#...

> Android P strengthens privacy by limiting the ability of background apps to access user input and sensor data. If your app is running in the background on a device running Android P, the system applies the following restrictions to your app: Your app cannot access the microphone or camera.