As rharve showed the window for playback devices, open instead the window for recording devices. Right click in an open area inside that window, you should then see a dialogue for hidden devices, put a tick in the box if not one there.
Providing Windows hasn't removed the facility in W8.1 you should hopefully then see a new device called "Stereo Mix" Right click on the "Stereo Mix" and choose "Enable" if it shows as disabled.

Next is look in Audacity, you have a drop down menu for the input, microphone etc, you should then see the "Stereao mix" in that drop down, choose that and see if it will record then.

It all hinges on whether the stereo mix has not been removed as a hidden device.