The problem of course, is that manufacturers don't let the hardware switch between the headphone jack and the internal speakers. Instead, they rely on the software to do it for them. I had the same thing with my intel HDA, and the only thing that worked for me was manually turn down all volumes except master, pcm and surround, which controlled the volume of the headphone jack.
Honestly, I think Intel deserves a mighty slapping for that whole intel HDA thing. I've had so much trouble with it in Linux, both on this laptop and a previous one..