Do you have another digital source to tell if you have the SONY set correctly to decode for your surround speakers? The digital out should be passing the signal.
In windows, right click the volume icon. click playback devices. Make sure the output device is set to the optical out. In the lower left is the configure button that would let you select the 2-channel or 4-channel or 5 or whatever if you were doing the decoding in the PC, but you are not so the configure button is probably greyed out.
Soem cross checks: Connect an HDMI cable from the PC to the Sony and disconnect the optical cable. That will send the same signal over the HDMI as your were sending over the optical. If it works now its in your PC settings. If it doesn work then it's likely in the Sony settings. The HDMI audio does not use your MB audio chip, it uses logic on the video card.
Here is some sony debugging. It might help: https
/us.en.kb.sony.com/app/answers/detail/a_id/31317/c/65,66/p/46871,74233,75026/