After some experimenting, I found that the problem was solved by disabling the microphone.
Right click (long press) the volume button in the taskbar and select ‘Recording devices’. Then right click ‘Microphone’ and press ‘Disable’. This was enough to solve the problem for me.
If you need the microphone again, open ‘Recording devices’, right click anywhere in the empty list and check ‘Show Disabled Devices’. Now you can enable your microphone again, but note that the volume fluctuations will be back.
Hope it works for you too!
Edit: Actually, you also might have to go to ‘Communications’ in the Sound settings (a tab in the ‘Recording devices’ window) and select ‘Do nothing’.
