Why does sound come speakers and headphones at the same time?

I have a Lenovo 3000 n100. I did a replacement on the motherboard. I installed a fresh windows 7 and updated all the drivers. But I have a weird problem with my laptop. I am not getting any beeps when the computer boot and no sound at the speakers while I am playing something in windows. You can see the signal on the tray but no sound. The weird part is that when I plug in the headphones jack, speakers and headphones work simultaneously. If I take off the headphones jack, there is no sound again. How could it happen?