Sorry about the delay, ISP upgrade to zero service and had to wait.
Stereo mixer and stereo mix are two different things. The first usually refers to an equalizer and the second to an audio input type. Now I know what you are talking about!
If you have the Realtek HD Audio Manager icon in the tray, double-click, Mixer tab. That should have been installed with the motherboard.
I couldn't find Realtek in my Programs menu, perhaps you can. Otherwise, Control Panel > Realtek HD Sound Effects Manager. In Program Files > Realtek > Installshield > double-click rtsndmgr.cpl will also do it.
If you do not normally have the icon in the tray, click the "i" information button once you have the panel up and select "Show icon...."