In windows 8 right click the sound icon in the notification area and select Recording Devices. In the pane that opens right click a blank area and tick both boxes. Hopefully you will see Stereo Mix. That needs to be the default to record sound via Audacity. For mic and Line in then change the defaults when they're plugged in.
Best Cyber Monday deals 2015 live blog: Best UK Cyber Monday tech deals, sales, discounts & offers