i am guessing that you want to get sound from your pc to play over your home theater? (otherwise you would just be connecting sony > tv)
the hdmi jack on the sony is for outputting video signal and is not for receiving an input from the pc. as far as i can tell it does not support hdmi (arc) either [audio return channel] which means you're completely SOL when it comes to 5.1 input. in short: cant be done unless it (and your tv) had arc support.
your dvd player does have stereo input which means you could play stereo sound from your pc over your home theater but i do not think this is what you were looking for.
unfortunately this problem plagues cheap HTIB [home theater in a box] systems and there are at least a few of these posts...