First, put your dual core back inside your computer. Then insert your headphones into the front audio port of your computer.
I joke. I understand what you mean. Checklist of solutions:
-Are you plugging it into the front mic port instead of the front audio port? Try the other one.
-Is the front post connected to the motherboard and are the bios setting set correctly for either AC97 or another setting?
-Do the earphones themselves work? Check with another computer or device.
-Are the settings in windows correct? Is the volume up enough?