I found a computer at work that had Realtek HD on board, so I enabled it so that I could have a look.
I think there are many variations of this, but here's a screenshot of the one I tried:

- RealTek HD Stereo Mix.GIF (54.7 KiB) Viewed 24337 times
To access this control panel, I double clicked on the speaker icon in the lower right corner of the screen.
Note that I have selected the "Mxer" tab.
The upper half shows the "Playback" settings and the lower half shows the "Record" (input) settings.
By default, many of the sliders are hidden.
To make the hidden sliders available, click on the "spanner" icon near the "volume knob".The left and right chevrons "< >" that are at the bottom right of the playback section allow you to scroll through the available sliders. If there are too many sliders to fit on the screen, then these chevrons appear.
You will also note that there are no sliders in the record section, but level adjustment can be achieved by "turning" the Record Level volume knob (click on it and drag)
Note the red crosses on the speaker icons in the Record section - these inputs are muted. In this screenshot, all inputs are muted apart from "Stereo Mix".
I think it is possible to enable individual sliders in the record section, but I didn't have much time to play with it (I was supposed to be working).