I bought new headphones(HD219 SENNHEISER). My computer has to units in which I can plug headphones.(one in front of and one back). I want to use in the front one since I have speakers at the back. The problem is the front one does not work.(Headphones work if I put them in the back one). When I connect them in front unit the computer register them, but there is no sound. I tried to open computer, but there wasn't much I see or could do there. Options as far as I find out are okay? Can somebody give me any advice what can I do, since computer register them, but there is no sound?