Hi all,
After much testing regarding this issue, we found a very simple workaround (at least with the Denon receiver tested here). In Control Panel > Sound, open the properties for the receiver on the Playback tab. Go to the Advanced tab and disable both of the Exclusive Mode options you see there.
Now check how the Dolby Digital 5.1 behaves. On our lab system, it immediately solved the problem. I'd like to get confirmation from others before I add this to our troubleshooting documentation on the support website.
Lois