There is a setting for eARC (Audio Return Channel) passthrough in the ATV. Then with a box like this you can have the separate audio output without the TV.

https://feintech.eu/en/products/feintech-vax04101-hdmi-earc-pass-switch-4x1-for-soundbar.
There are lots of other eARC passthrough boxes that will do more or less the same thing. This one from Feintech has worked really well for my setup.