The 5.1 digital output is source based. Your blu-ray player can only output 5.1 digital if the source disk is encoded with a Dolby AC3 source and the player itself is simply passing AC3 through to the optical port. Like for menus and player sounds from the blu-ray player is always stereo. A PC won't support 5.1 Dolby output unless it's also simply passing the signal through. The PC itself is incapable of muxing 5.1 channels into the optical digital stream, that requires a licensed Dolby encoder which no PC has. A game might support 5.1 Dolby output but only if it's paid the Dolby 5.1 license, it will have a Dolby 5.1 logo if it does which most don't and even then, the only 5.1 content is movies already encoded in 5.1 AC3. At best, most games and PC content can only do stereo but some can pre-mux in Dolby Pro Logic and even then, the PC is still only outputting stereo that requires a licensed chip or software in your receiver to decrypt the Pro Logic surround information.
