The Mic is normally activated when Voice Services are setup, there is no option to turn the mic on or off via settings due to privacy concerns. I suggest you Disable then Enable the Sonos skill in the Alexa App>Skills>Your Skills and then retest
If the problem persists, then please Open the Sonos App on a mobile device and select … More>Voice Services>Amazon Alexa and follow the steps.
Thanks for the reply.
I tried disabling and then re-enabling the the Sonos skill in Alexa, but no change.
When I go to ... More > Voice Services i see what I suspect could be part of the problem: A blank screen that reads "Not available in your country" (screenshot attached).No other options to select except "Done."

I happen to be living in Japan, but since I'm setting this all up in English I'm not exactly sure precisely "what" is not available...
Also not sure how that affects usage of the mic.