I have a Dell Latitude D620 and a D-Port replicator and had never succeeded in getting the sound jack on the dock to work. I found a solution on Ubuntu forums: http://ubuntuforums.org/showthread.php?t=80193
Its quite simple. You just have to enable IEC958
Open Volume control
If you dont see the “Switches” tab:
Click Preferences and check “IEC958″
Open the Switches tab [...]