This is so simple, though many people tend to ask for it. Whenever your Skype rings you notice every sound of your system to reduce by 80%, that's a Windows OS option which you can easily change. Follow the steps below:
Start Run and type mmsys.cpl.
Go to Communications tab and select 'Do nothing'.
From now on your windows will not reduce sound whenever a call is happening on Skype or any other communication software.
