I use Windows Media Center in Win7-64bit. I watch DVDs and Netflix without any audio issues. The fact that you're getting audio from Netflix suggests it's your HDTV that is not recognizing the audio signal from your DVD.
If this is the case, you'll need to look at incompatibilities between the DVD itself (what audio signal it's encoded with), what audio signal your sound device is sending to the HDTV, and what signal type your HDTV can accept from that specific input
What I'm trying to say (and this is just as an example. I don't know what stream Netflix uses or what the DVD is trying to stream) is maybe it's something like Netflix streams a 2.0 audio source whereas the DVD streams a 5.1 audio source. Your HDTV can handle and does play the 2.0 source, but it can't handle the 5.1 source, so it doesn't play that at all.
Again, just an example. I don't know what each audio source is sending to the HDTV, but it might be something to look into.
Ok, I think you're a bit confused. The PC outputs both sound via 5.1 onboard sound. There is no sound going to the HDTV. All sound comes from the PC just like all video comes from the PC. HDTV should only be showing the video not trying to output the sound as the 'onboad' sound should play the sound. It's like Media Player can't find the codecs to play the DVD's sound.
Or maybe I'm the one confused...
Thanks for trying to help though, it's appreciated.