I take it your headset is not optical, but your speaker-system has an optical input?
The sound jack is a normal wired thing, you have an adapter which includes an optocoupler (say) which turns the electrical signals into light.
If all of the above, then, this probably means the adapter is broken or your speaker system isn't switched to receive signals from the optical input. If the speaker system has a regular input, try that (at least the speakers are going). You don't get any benifit from optical transmittion (timing, attenuation, etc) unless the speakers are a very loooong way from the computer. Like the next building.