If you plug it in and it works intermittently then I believe there might be a problem with the acutal cable going to the sub.
It happened to mine
Anyways can you plug the jack into a Ipod or MP3 and give it a shot?
He has tried that, by plugging into another computer, with success.
The new sound card rules out the bad old one...
The wire is always suspect, they become picky as to what angle a bend in the wire can be for it to play correctly, after being damaged...i have headphones like that..perhaps u got lucky with the other computer. Try moving the cords around, pinching it slightly around the base of the headphone jack. turning it around. You will notice what i mean if that is your problem, the sound will come in an out.
I dont know what would be wrong in the computer, you seem to have checked everything, but take a look into the device manager and see if anything is wrong there.
Edited by s1lents0ul, 24 March 2011 - 06:18 AM.