The PC I bought has "ONBOARD 8 CHANNEL (7.1) HIGH DEF AUDIO (AS STANDARD)". Will check the connections
Have you got a dedicated soundcard? Or onboard sound only? Does the ports on the back work for sound?
It could be that the headers to connect the ports on the front to the soundcard or motherboard are not connected. If you take a look you should be able to see the cables coming from the front and connected to wither your motherboard (if you do NOT have a dedicated sound card) or to the soundcard. If you can find them, check to ensure they are properly seated, remove them and re-seat them if required. Then try again.