System: LG UHD display, Apple TV 4K (A12 Bionic version), Sonos ARC SL/Sub/Play1 home theater
Problem: volume control stopped working / no audio output
Several days ago I noticed an issue with my Sonos home theater - no sound output in Prime videos. Clicking the volume control on the aTV remote brought up a grey circle with a slash on the right edge of the screen. I was still able to get video out of locally served movies as well as the Hulu app. Later in the day, the Hulu sound quit, followed by Apple Trailers. Now I have no sound at all, even from the aTV click and window system sounds.
I discovered that none of the apps on my devices could see the Sonos system (once the Sonos HT was installed I never again used the apps), so I did a WiFi reset as prescribed in the support docs. My apps could then see the system, but the audio was still gone. I reset the remote function and had the app relearn the aTV remote, still nothing but that grey circle with a slash on the right side of the screen. However, I was able to control the volume through the S2 app.
I then updated the app, and also did the firmware updates on the HT components. Still no audio control via aTV remote.
What is going on with this? Why did the volume turn off selectively and progressively across the Prime and Hulu apps instead of all at once? What is that grey circle with the slash through it?