I have it hooked up via an optical cable. (722 to Onkyo 505) Here is what is happening: Sometimes Discovery HD transmits DD 2.0 as 5.1 and my receiver does not know what to do with it. Therefore, it only plays sounds out of the left and right speakers. There is dialog, its just not coming out of the center channel (No surround either!).
I have had this same problem for quite a while with other channels too. Mostly my local ABC and NBC (and now SciFi HD). When tuned to a HD channel, and a HD show is on, the sound would be in 5.1 and sound just fine. As soon as a commercial would come on, or a non-HD (SD) program, the center would disappear with a ghostly hollow quality. However, I've never seen this problem with CBS HD.
But, the other day I stumbled onto a solution. I have a Yamaha RX-V630 receiver hooked to my 722 with optical cable. The Yamaha is suppose to switch automatically between 5.1, Dolby 2.0, etc when getting digital sound data. When switching between true HD and SD channels, this sound switching would always work correctly (except for the annoying volume difference. Another story and thread). However, when SD or commercials (which are largely still in SD) would come onto a HD channel, the sound data actually changes from 5.1 to Dolby 2.0. My receiver wouldn't switch and try to decode the Dolby 2.0 as 5.1; and created the funky sound. But as I noted above, not with CBS HD. That channel would always switch correctly.
This is why I kept both the HD and SD versions of channels in my favorites lists, so that when I watched HD shows I could hear proper 5.1, and when watching SD shows I could hear proper Dolby 2.0 sound.
I always kept my receiver in one of it's DSP processed "Sound Fields", I think it was called "SciFi Enhanced". This was to (as the manual said) "enlarge" the sound field (it didn't seem to sound any different, but I kept there just in case). But somehow this DSP field mode was keeping the receiver from switching from 5.1 to Dolby 2.0 properly; but only when tuned to some HD channels! Weird?!?!
I found that if I turned off ALL of the DSP Sound Fields, the receiver would start properly switching between 5.1 and Dolby 2.0 when tuned to any HD channel. And to my pleasant surprise, the sound volume DIDN'T change! The SD/2.0 just wasn't as rich as 5.1 sound.
Here's what I think is going on (though I don't have any technical evidence to prove this). When tuned to a SD channel, the sound is tagged as being Dolby 2.0 sound, and the sound switching works correctly. However, some HD channels seem to blindly tag ALL sound as being 5.1, regardless of it's HD or SD origin. This "tricks" audio receivers into try to doce Dolby 2.0 as 5.1 sound data. CBS seems to be the exception, by either tagging Dolby 2.0 properly, or "converting" Dolby 2.0 to 5.1 before transmission. The Yamaha receiver seems to be just a little bit smarter (when not in a DSP Sound Field mode) in that even if it's being told that sound is 5.1, it looks at the actual data format and figures out it's really Dolby 2.0 sound instead.
Now SciFi HD is a pleasure to watch, and I've taken out all of the SD versions of HD channels out of my favorites lists.
You might see if similar configuration changes of your audio receiver would solve this issue for you also.