Hello Thomas, welcome to the Seven Forums.
Not knowing which audio devices you have, I can not tell you which device you have to choose, but in my opinion this can be done from Skype Audio Settings.
An example from my laptop with IDT Audio chip. I can select one of these devices:
Selecting Stereo Mix in my case sends the signal from mic when it is connected but when mic is not connected, it sends whatever sound I'm currently playing, for instance WMP, WMC or speech synthesizer.
With an image it's diffult to show audio
but I'll try: screenshot below shows (green signal strength bar) Skype is getting an audio signal to send. All mics currently disconnected, that signal comes from a YouTube video I'm watching, so if I now had a Skype chat with you, you would receive and hear what I'm listening now.