Skype 4 Business does automaticaly adjust your microphone volume 'for the best experience'.
There is an official workaround to disable this feature but this does not work on my side.
(Checkout this here: https://support.microsoft.com/en-us/help/4054427/disable-automatic-gain-control-skype-for-business-windows)
UnfuckMicrophone is a background process that maintains the main microphone volume to 100% to fix Skype 4 Business microphone mess.
For Windows only.