Yes. It’s the pause button. Pausing is not the same as muting, I know, but this is the way to silence your Sonos. If you use Google Assistant on your Sonos say “Hey Google, stop”.

As a further detail,  if speakers are grouped then a short press / tap will pause play on all the grouped speakers. A long press will ungroup that speaker and so stop the music on that speaker only.

Sorry, should have mentioned that I want to only mute one speaker in a group. I guess then the answer is there is no mute, but a long press is the closest option.

That's correct. 

I use long press quite often.  If I am moving from one room to another I'll stop the music in one room then long press in the next room to pick up the music. Or leave music playing in the first room and long press in second room to group the two rooms.