Hi,
I had a similar setup until recently. As Louie says, the windows audio drivers have no way to read EDID information indicating the 5.1 audio capability. But with XBMC - you can configure it to do pass through audio and the soundbar should be able to decode the audio properly as 5.1 audio. See: Settings/System - XBMC
- the 'Passthrough output device' part.
I think the only way to get windows (with the drivers) to do 5.1 audio is through HDMI into a receiver or a TV that has support for it directly. This is because the audio driver has to send the audio raw and uncompressed over HDMI for each of the channels (I believe in the case of the optical cable the audio must be already encoded as DTS or Dolby).