I had the same issue.No option to change in keyboard settings,google voice typing,etc.Installed Googles keyboard and while it shows a mic button ,pressing it results in "Google voice is not supported".Tried Googles assistant "Not supported" and Tried to rerecord my voice in Google settings "Not supported" All these functioned untill 2 weeks ago.Longer story shortened,I reset my app prefences,in turn Agreed to Permissions and recalibrated My "OkGoogle " +("HeyGoogle").Everything is back to as it was.Microphone icon is back and functions.I really hope this helps.