Can't be done. Sorry.
What's insane: Guess what happens if you hold the remote microphone button and ask Siri to Mute?
Some responses are:
"I'm sorry, I can't do that for you"
"Use the buttons on the remote to turn down the volume"
Hello plusd,
Thanks for that info and for choosing the Apple Support Communities. If we understand correctly, you are unable to mute volume on your Siri Remote. To do this, please follow the relevant steps in this Apple resource on this situation, and then contact Apple if you need any further assistance.
Control your TV or receiver with your Siri Remote or Apple TV Remote
Cheers!
I have been complaining about the lack of a mute button on ATV remotes for years. How difficult would it be to make a double click of the "-" volume button a mute function?
Don't expect too much but you could submit this suggestion to Apple Feedback:
Can't Mute TV using Apple TV Remote
