I have a AOpen i945GMt-FA mini-itx motherboard and I'm using onboard audio. How it works though is that there's an optical adapter that goes into the normal sound out port. The sound works when I plug my headset into the port, but I have absolutely no idea how to get it to work with the optical. Does anyone have a similar situation or can point me in the right direction?

This the one?

I don't see anything about an optical adapter here though... it seems to have the usual jacks.
What are you trying to do with the sound-out port that requires a special adapter?

Yeah it doesn't really mention the optical out. But there is an adapter that goes into the line out port. I want to connect the audio to my receiver optically.

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.