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

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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