My company bought a bunch of Dell optiplex desktops with Realtek HD Integrated Sound a year ago and they've all got the Stereo Mix disabled. I did some searching on Google for a solution and found what you need to do.
Download the SoundMax driver from Dell's website and extract it somewhere. Then open up ADIHDAUD.INF located in SMAXWDM folder in the driver folder.
Find the line HKR,AD1984\\Disable, OutR, 1, 01 ;; Disable Stereo Mix Capture Node
Change the “01” to 00 and save it
Run “setup” and select “uninstall” existing driver - reboot
Run “setup” and select “install” new driver - ignore the Windows XP auto install setting and ignore the “non certified” driver warning - reboot
Go to start menu > run > type: “regedit” [enter] (without quotes)
Click on “edit > search” and look for AD1984.
For each registry key/folder of AD1984, find the subfolder/key that says MicBP and OutR and change their associated binary values from 01 to 00. For some people, re-installing the driver in this fashion already produces a 00 while for others, the change must be made manually.
Reboot
That should allow you to use Stereo Mix like normal.
beef_wellington
I have a HP Pavilion DV 9000t (Vista). I had to manually re-enable mine as follows:
Right click Sound icon,
Go to recording Devices,
Right click the blank area,
Select “show disabled devices”,
Right click on “WaveOut Mix” and select Enable.
There you go.
jakthebomb