Hi respected Simon Schwartzman,
Thanks for your valuable feedback and sharing updates with us and I really appreciate your effort and your precious time doing.
And I appreciate your kind words.
Could you please verify that Word has the necessary permissions to access your microphone. You can do this by going to System Preferences > Security & Privacy > Privacy > Microphone. Ensure that Microsoft Word is listed and enabled.
If still doesn't work, remove the Office folders from Containers folder in Library with following steps.
a: Completely quit all Office apps
b: In Finder, press COMMAND +Shift+g.
c: Enter**~/Library** and click Go
d: Go to Containers folder, find the folder called Microsoft Word or com.Microsoft.Word
e: Move this folder to your desktop by cut&paste .
f: Relaunch Word for Mac application and see the results.
Also, try to reboot Mac in safe mode and determine whether background programs are causing the issue. For detailed information, please check this article.
Disclaimer: Microsoft provides no assurances and/or warranties, implied or otherwise, and is not responsible for the information you receive from the third-party linked sites or any support related to technology.
Meanwhile, sign in www.office.com and use the dictating in Word for the web to check if the behavior on online side.
If my above suggestion doesn't work, and since you are using Office 365 insider version (16.75), we will move this thread to correct category for further help.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Sincerely,
Waqas Muhammad