It switches by itself.
It switches according to what Skype wants. Skype became the most reliable and respected communications program on earth by being vicious and brutal about getting its way.
We recommend using a Skype-compliant program to record your works instead of trying to use Audacity. In a contest between Skype and Audacity, we haven’t got a chance.
Pamela.
http://www.pamela.biz/en/products/
I think Voicemeeter can do this.
http://vb-audio.pagesperso-orange.fr/Voicemeeter/banana.htm
Other offerings.
Koz