Steps:
- Open the Control Panel (not to be confused with "Settings"), click "Hardware and Sound", in the Sound section click "Manage audio devices".
- In the Sound pop-up click on the "Recording" tab, right-click, and select "show disabled devices".
- At this point "Stereo Mix" should be listed if it's supported by your sound card driver. Right click on "Stereo Mix", and choose "Enable".