You should be getting digital sound from the TV to your surround system via the HDMI (I believe).
It's possible the source is not Dolby Digital, so to get surround sound you would have to switch the audio system to Pro Logic mode?
As for the Xbox 360, you'll need to buy (if you don't have one) an optical cable and run that from the optical output jack that's found at the beginning of the Xbox 360's component cable attachment.
It's beside the cord on the square bit that plugs into the back of the Xbox 360, there's a spot there for the optical cable.
If your Sony surround thing has more than one optical input on the back, you could also run an optical cable from your TV to your surround system if you really aren't getting Dolby Digital from the HDMI (though I suspect it's the TV broadcast not actually having Dolby Digital). Otherwise you can just use regular analog cables and only have stereo or Pro Logic.