Try this… in the Harmony app, go to Harmony Setup > Add/Edit Devices & Activities > Remote & Hub > Button Customization > Activity Control Buttons > Select Activity and customize a button on your remote to mute your Sonos One.

Thanks, good workaround and it works well.

I would hope there is a config somewhere that I might be missing that mutes the TV signal or perhaps the Mute button should mute all volume output on any devices.

You could also customize the Mute button to mute the Sonos One with a long press while it still mutes the TV/Playbasewith a short press.

I experimented with your approach and ‘thought’ I might have it but no luck.

1. Customized the button to Sequence instead of command.
2. Sequence 
   1. Mute Sonos Device 1
   2. Mute Sonos Device 2

I would have expected the Action would mute both devices, but it Round robins between them on each button push. I added a 1 sec delay between them but no luck. Any ideas appreciated.

GOAL: on ‘Mute’, mute Playbase and Sonos One together as one, as both were activated with the Harmony activity ‘Play Tv’.

Disregard. Sometimes you just need to delete the activity and re-create the activity from the beginning, which worked. The mute button is muting both Sonos devices.

I think the reason why your mute button is only muting the Playbase is because the Harmony is actually set to mute the TV, not the Playbase. And since the Playbase is directly connected to your TV via optical and your Sonos One isn’t, it only mutes the wired speaker.