“I’ve tried  just about everything” is not really helpful, if you’re asking for help, as we can only guess at what you have read and tried. 
 

Power-off reset? Factory reset? Toggle speaker on/off in app? LED toggle on/off? Sonos voice control? Other actions? 

Sorry yes.. just going a bit crazy.. tried powering on & off numerous times, waiting various lengths of time too. Tried  removing Alexa skills and re-enabling them. Tried to deregister the device on Amazon and re register . On the Stauts bar within the app the mic is light grey and crossed out, but obviously will not just toggle on&off as I would have expected.. I have no issue with my Beam & Move all working fine.. and the One was ok up until 7-10 day ago- I have auto software on, and thus show S2

Does Sonos Voice Control work on the others (are you able to add that)? 

Do other controllers show it similarly disabled (can’t remember immediately if the windows or mac app would show those options as well). 

I suspect you’ve read this older thread which sounds similar and had some suggestions to check whether the touch controls on the One are disabled etc.

Yes, Alexa voice l control works fine on my Beam & Move, but just stopped working on the One a week or so ago.. the microphone symbol is crossed out on the Sonos app .. 

I’m wondering whether a factory reset might help, since you have other devices in the system. Maybe generate a diagnostics report before doing so. What do others think? 

This was going to be my last resort, but I keep seeing warnings against it, unless absolutely necessary- not sure  why it’s seen as being such a big deal- but people warn against it all the time. I have no real customisation on the One that I’m fearful of losing- so unless they answer the phone or call me back, I will bite the bullet in the next 48 hours or so! I need to chill first as it’s stressed me out so much already … then I remind myself to get a grip - it’s only a First World problem

I wondered if you’d be able to add this: https://support.sonos.com/en/article/set-up-sonos-voice-control

Likely you should be able to on the others but if it’s that the microphone is disabled on the One then it very likely won’t let you add the Sonos Voice Control on it either. 

Likely your devices have the same firmware but is there a way to see if they all have the same firmware in case that’s causing it? 

https://support.sonos.com/en/article/check-system-information-in-about-my-system

A strange update on my issue for those interested- my Sonos one allowed me to add the Sonos voice control (in addition to Alexa) and with this the microphone works and the icon itself is illuminating on the One product- the app is telling me Alex is installed, but it does not actually pick up any instructions/ skills - so I can use Sonos voice control at least, but not Alexa. All my other products are unaffected and although it gives me an option to play something for dog as a leave the house, it’s still not functioning as it did before the S2 update- he was a fan of greatest hits radio, sadly no more

 Do you see the One in the Alexa app?

There have been a few posts about problems with Alexa and where factory reset has helped. Could it be the same as discussed in that thread that Sonos thinks Alexa is on your One but Amazon thinks it is on a different device. 
 

I nowadays use mainly Sonos Voice Control but need to use Alexa when I want to start a particular Sonos Radio station: there are two with exactly same name and Sonos Voice always starts the wrong one. Alexa starts the one I want. 

List of Sonos Voice Control commands:

https://support.sonos.com/fi-fi/article/list-of-voice-requests-for-sonos-voice-control

Same issue here. Problems started after Sonos updated their Android app on Aug 25, 2024. The microphone has since been greyed out on the app, microphone light does not do anything on the unit when I press it, Alexa/Google Assistant stopped working, can't add voice assistants (not Sonos, not Google, not Alexa). I tried a factory reset. No dice. Ideas?