Go into the “Sound” control panel applet, (e.g. Start -> Run ->
select the “Recording” tab,
Then right-click on and enable the “Stereo mix” device.
(If “Stereo Mix” isn’t being shown, then right-click in the white space or on one of the listed devices and ensure that “show disabled devices” is enabled)
You should now be able to set that Stereo Mix as a source for recording. i.e. it should show up now as an alternative “input” or “microphone” device in the app you are using.