I would recommend that you submit a system diagnostic within 10 minutes of experiencing this problem, and call Sonos Support to discuss it, especially as it is possible, although not likely, that there has been a hardware failure. 

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your network and Sonos system.

Ok. Thank you for your advice

Having graduated from a live chat to a call to Sonos support, it appears there is  currently a major fault with Alexa voice services which Sonos are trying to fix. This coincided with my software update problems and having to reset and create a new account, the final part was to add a voice service before this allows my microphone to function. I've  been informed to wait until the outage is resolved and then try again. If it still doesn't work I'll give the support team another ring.

After a factory reset, you need to add the Alexa voice assistant to the Beam again.

1. Open the Sonos app for iOS or Android.
2. Go to the Settings menu and select Manage next to My System.
3. Select Voice Assistants, then Add a Voice Assistant (Alexa).
4. Select Sonos Voice Control.
5. Follow the in-app instructions to add Sonos Voice Control to your Sonos system.

I have spent several hours trying to find a solution to the microphone problem on my Beam, (including long conversations and help from Sonos Support, as well as seeking advice from the Community).

The problem has resolved itself with the latest revamp of the Sonos App!

Mine is doing the exact same thing. What was the fix? The microphone light isn’t responding to touching it on and off after the factory reset