mute doesn't doesn't mute

Do you have this remote?

The Mute button would work out-of-the-box for many audio systems, using the most common volume control signals, including for Sonos. Your TV would normally relay the Mute command (and Volume Up/Down) via CEC. Else it has to “learn” a remote.

Can you direct me to a specific setting that might enable 'mute'?

There is no such setting. It should work automatically when configured correctly.

Sonos volume control should work via HDMI-CEC. Is your Sonos connected via HDMI? You would have to activate CEC on Apple TV [1] and on your TV [see its manual].

Does the mute button of your iPhone's Apple TV Remote (launched from Control Center) work?

Have you tried resetting and re-pairing your remote as described in the "Related Article" listed at the top of this thread (If your Apple TV remote isn't working)?

FWIW, I've never "programmed" my remote to mute my Sony soundbar (controlled via HDMI CEC). I actually have the older (minimalist) black Siri remote without the mute button, but the mute function of the soft remote (on my iPhone) works correctly.

Some audio systems may not support the Mute function.

Some audio systems use signal commands that are different from the pre-programmed default. If so, then you can learn it a new device. Settings﹥Remotes and Devices﹥Volume Control﹥Learn New Device…﹥

So we're optical instead. Audio quality is fine.

Then you may have to learn the remote to send a Mute signal that Sonos listens to.

Does it work with the Mute function on your TV remote?

If HDMI was the issue, the volume toggle on the remote probably wouldn't work either, right?

I would expect Volume Up, Volume Down, and Mute to work “all or nothing” either way.

Sorry - I just noted your use of optical connection. The iPhone remote won't control volume/mute since IR's your fallback control method for the Apple TV Remote. Again, reset the remote and train it as necessary. I assume you are given the opportunity to teach the mute function as well as volume up & down?

Also, are you sure you told the Vizio to output audio via HDMI ARC? Simply plugging the Sonos into the HDMI port labeled ARC usually isn't enough :-).

It's likely that the TV isn't showing HDMI ARC as an option because you've disconnected the Sonos from the TV's ARC-enabled port (this indicates some sign of life on the port in question). If it were me, I would disconnect the ATV4K device (and any other HDMI media devices) from the TV entirely and then see if I could get just the TV and Sonos working together via HDMI by enabling HDMI ARC output and CEC using the TV's setup menus. Play any TV-based content to test. If there's a way to RESET the TV in this respect, try that. Also mess with the sonos settings for enabling the HDMI ARC & CEC connection as needed.

2015 is semi-old by home electronics standards... have you verified that you're running your TV's latest available firmware for your model? Likewise make sure the Sonos is current.

@MrPheasant wrote:

Ultimately, we probably need a new TV.

I was thinking the same thing :-). You can pick up a new TV that's much better than your 2015 Visio for a fraction of what your Sonos setup goes for. Your TV is clearly the weakest link.

You would access this Setting on the Apple TV box, to change the behavior for the physical Siri Remote.