In this case it seems that the volume is controlled by an application. In order to turn this off you must locate the applications which use the MIC (Skype, a generic messenger, etc). These applications are known to handle MIC settings when they are installed on the computer so the best way to fix this is to go into the program's settings > Audio > Microphone. From that page, locate "Let handle microphone audio". Uncheck or turn off the setting and restart your computer. You will notice that the volume of the MIC remains the same and does not automatically adjusts.