On iOS 14.3 and tvOS 14.3 on Apple TV 4K you can only add one or one paired set at a time in the default audio setting on an Apple TV 4k for the “Home Theater” mode. Once you’ve applied that setting on the Apple TV 4K you will override the AirPlay function in control Center and cannot remove any you added . Set up home theater audio with HomePod and Apple TV 4K - Apple Support
If you want to play to more than one HomePod or more than one pair, it’s best not to use that setting - so set that to tv speakers in ATV Settings > Video and Audio > Default Audio Output. Then you can go ahead and AirPlay to as many as you like using control Center like this:
- Play the audio that you want to stream from your Apple TV, then press and hold Apple TV App/Home to bring up Control Center.
- Select AirPlay .
- Choose the headphones or speakers that you want to stream the current audio to.
I’m not sure how the default audio setting works on Apple TV HD (if it overrides control Center)
Or you can use the swipe down method to AUDIO to make your selection
Until such time they issue a software update to fix this, this is how it now is.
I hope that explains it.