This solution is useful to restore the deprecated stereo mix from modern Windows machines –
Install this small application VB Audio http://vb-audio.pagesperso-orange.fr/Cable/index.htm
In Control Panel, go to the Playback tab and make the “Cable Input” virtual device the default audio device.
In order to hear audio through the device on normal stereo output, through the audio jack etc. right click Cable Input under the Recording tab (check this is also set to default), then go to the Listen tab, make sure “Listen to this device” is checked, set the playback to a valid speakers on the PC.
The above can now be used to record audio playback from the maching, e.g. in Audacity make sure the ‘Proterties’ > ‘Devices’ area is set so ‘Playback’ and ‘Recording’ are both shown as ‘Cable Input’.
The above method also allows for listening to audio after recording.