Audio Equipment and Home Theater Audio : having problem wiv PC speakers

having problem wiv PC speakers

i have just fitted a front Audio/USB port on my PC, i have plugged in the cables onto my Asus Motherboard and they work fine (mic and headphones) however when i unplug the headphones my rear speaker socket doesnt work anymore, the rear sockets are onboard (Soundmax) anyone got any ideas on how to get my rear speakers working when i unplug my headphones from the front plz
thanks in advance