Right click on the sound icon in the taskbar, a menu will pop-up, choose
recording devices.
If Stereo Mix is listed, high-light it and click on properties and the prop
window will pop-up.
At the bottom of the General tab there will be a fold out menu titles Device
usage. It will probably say disabled, click on the down arrow and choose
enable.
Click OK all the way through.
Now, the other question is which Audio software do you have? If you have
RealTek then you will need to open that up, either through the brown sound
icon in the taskbar if its there or through the control panel.
In the top left corner you'll see 'Device advanced settings' click on it and
in the pop-up window choose 'Separate all input jacks as independent input
devices'
Click Ok and let the manager do it's thing. When done you should see a
Stereo Mix tab.