If you have sound from your headphone socket on your computer your sound card is working, so I think it is your equipment that you are plugging into the computer that is faulty.
The light on the speaker system only shows there is power coming out of it's power supply, this does not show faults elsewhere.
Check all of your cables for signs of damage, if none found it is more than likely a circuit fault inside your external speaker system,,, not really worth repairing, cheaper to buy some more.
Cheers Shewy