Huh. I had the same type of stuff. I never figured it out.
When I go into sound prefs and select which output I want it to play out of, it works just fine. But if I plug in headphones then, in the front jack, it won't play sound through it like it did on my G4 tower. I have to go into sound prefs AGAIN and change the settings.... My G4 did it automatically, and it could play out of the speakers and the headphones at the same time!
Can anyone help me?