It's somewhere in the input settings. I have a 919 at home, and haven't messed with the settings once I set them up initially. As far as I can remember, you have to set up the HDMI input to which you have connected the STB to. For eg. if you have the STB connected to HDMI 2 input, then go to input settings (I think it's one of the options on the Home Menu), then go to the HDMI 2 input, then if you go down, there's an option for selecting the Audio IN. You have to change that to OPTICAL instead of HDMI.
This is as much as I remember from early 2010 when I set it up. I might be wrong, but I think that's how you make the AVR use video from HDMI and audio from Optical.
Hope this works.