Ok so I am using Windows 10 versoin 1511 Build 10586.218. My sound card is build in Realtek HD Audio.
Whenever, I use the mic, say for Skype call or in Control Panel>Speec Recognition>Setup Microphone, my microphone volume starts decreasing from 100 to downward. (Notice that it only decreases when the microphone is being used in some software. If I just speak to it in idle window, the volume does not decrease.)

Now whether I install the Realtek audio driver or not, it does not matter. I have also put "Do Nothing" in Sound>Communication.

Know that this kind of problem did not exist in my Windows 8.1.