This has happened to me too, around the same too. I'm pretty sure the setting didn't used to be called "Default Audio Output" either. I think it was just "Audio Output". I used to connect to the Sonos every time I turned on the Apple TV, and would have loved to set it as the default, but that was not an option. Now, because the Sonos isn't appearing in the newly-renamed (I think) "Default Audio Output" menu, I can't connect to it at all.
Really annoying troubleshooting this because when you search for it, these devices could connect with each other in so many different ways that it's hard to even find mention of this, but it definitely worked for me for the last year or two. It seems that they've removed this feature for some reason, whether by mistake or by design.
I'm having to find workarounds to watch stuff involving my cables and my MacBook Pro, which defeats the point of owning the Apple TV in the first place.
Hopefully they fix this or else a useful answer appears soon.