My microphone on my Apple Watch Series 7 stopped working? I have already re booted and re paired my phone. The microphone appears clean.

You could also un-pair and set up again but as a New Watch to undo any incorrect settings.

If you already completed that troubleshooting and the problem is not resolved then reach out to Apple Support

Click here > Contact - Official Apple Support  

or phone numbers  

Click here > https://support.apple.com/en-us/HT201232 

and or 

Genius Bar Reservation and Apple Support Options - Apple