I wonder if the setting and calibration for Wireless Audio Sync has gone horribly wrong?
Apple TV 4K > Settings > Video & Audio > [bottom] Wireless Audio Sync (recalibrate)
Revisit the other audio settings, perhaps a PCM or other protocol has faded away to the abyss
If you are using external Audio hardware equipped with codec's to translate multichannel support such as Atmos etc, revisit the settings in the same menu.
DRM might be playing a role? The content presented and displayed to your TV via the HDMI hose is under software observation, restart or reset might be in your future to solve? Reseat the power of your TV as well. Give it several minutes without power. While you are there if you have an external sound hardware, give a break from power at the same time.