Screen Mirror sound from films only comes out over MacBook Pro speakers
I have a 2017 Macbook pro running 12.5.1 and when I screen mirror or AirPlay to my Apple TV box the picture comes out but the sound is only via my Macbook's built-in speakers. This also happens when I try to use my AirPods and B&W P7's so abviously has nothing to do with either Bluetooth or wifi.
And finally even if i mute the internal speakers the sounds still come out through them even without trying to airplay or mirror. I have restarted everything!
[Re-Titled by Moderator]