DFI Lanaparty NF4 karajan help....

Hi I bought this computer about a year ago (sig...) and have been content with plugging my speakers into the front port of the computer however now i bought new headphones I would much rather use the back motherboard ports however when I plug into them I only get sound out of one side of the headphones or only one speaker. How do i change the normal audio from the port at the front of the case to the port connecting to the motherboard at the back? There is a cord running from the front port to the karajan audio module on the mobo and i tried unplugging that however that just killed all sound alltogether. Any help would be appreciated.

Thanks

Goose