longshot guess, but do you have to tell your sound card to use the headphone jack?
on my home computer when I plug in my headphone a program opens and ask me what I just pluged into the 1/8th audio jack. I check the headphone's box and then they work.
The program that opens is software from the sound card, not the built in windows audio controls - but the software controls those features (volume, recording, etc)
Perhaps you have something similar but the software isn't launching to ask what you just plugged in?