I asked a similar question a short while back, using XP. I got no answer to the problem.
But while experimenting I noted that on many occasions the Audacity mic volume control was linked directly to the PC's volume control that pops up when I click the loudspeaker icon at the bottom right of my screen -- i.e. I change the PC speaker volume and the Audacity mic volume moved in sympathy with it. Quite often, if I had changed my PC's speaker volume for listening to tracks, or to a soundrack on a web page, I would find that Audacity's mic volume control had changed to maximum next time I opened Audacity. And quite often, if I tried to change the Audacity mic volume it would snap-to maximum volume.
My only solution was not to touch the PC's volume control during a recording session where I wanted the mic volume to stay the same.