Is there a setting in the bios for enabling 5.1 as opposed to 2.1 Mustapha ?
You say that the rear channels are into LINE IN,that doesnt sound right
Can you tell us which mobo you are using so that we can see the connections available or put a link or a pic of the mobo on here to save us searching
Incidentally my speakers threw me when I got some new ones,only 2.1 for me but with a subwoofer,they were colour coded for the connections using the speakers and subwoofer itself as junction boxes but I hadnt noticed that at first