Firstly as has been mentioned Optical connections can only handle uncompressed 2.0 signals, but there are ways around this.
Is there an option in your drivers to encode DTS or Dolby Digital on the fly and send this to your receiver? sometimes you have to configure the driver to 5.1 and then select speakers instead of optical in windows playback devices in order for this to work. You also might need additional drivers or components/modules of a driver for this to work. I had to pay $3 to creative for the DTS module for my X-FI to do this, although I rarely use it unless I need to use the soundcard's features like Smartvolume now that HDMI is working properly.
This
link should have more info for your particular chip:
This will work for games but it won't be optimal for movies as you will effectively be compressing the already compressed movie soundtrack, basically making it go through DTS compression twice.
The best way to watch movies is by bitstreaming the original DTS/DD audio to the receiver for it to decode instead of the computer.
You can configure MPC to bitstream like
this: (I'd also recommend MadVR for visuals.)
Note you won't have enough bandwidth for any of the uncompressed HD codecs, you'll need HDMI for that.
You can also use a winamp wasapi plugin like
this to bypass all the windows re-sampling and get cleaner music to your receiver.
Make sure you tick all the boxes in the exclusive tab of the plugin options EXCEPT for the 2 resample options.