Not sure but try disabling the internal speaker in OSX and then booting into Windows and see if that changes anything. My MP is not here yet so I can't test it but this is definatly something I would want fixed because I will be installing Boot Camp on mine when it gets here next week.
I'm having the same problem. If I plug something into the front headphone jack, then it will turn off. I can't get it to turn off with speakers plugged into the back though. It's pretty noisey too, so it's pretty frustrating.
I had same problems since I am using digital audio out. So what worked for me in XP is just drag the "Master Volume" all the way down in volume control. Digital out ofcourse doesn't use this so internal speaker is dead silent and SPDIF keeps play'in