Well that is interesting.
My Sony receiver sometimes does not like to decode the DD5.1 so I have to change the output setting and then change it back. Not sure what you settings are with the Denon, but give that a try. On the Sony, changing it to "auto format decode" and then back to surround sound (sometimes more than once) often does the trick.
Sometimes a cleaning of the optical ends can help as well. Just a little windex on a paper towel...no major effort. Can you see the red light at the end of the cable that connects to your Denon.
I'm still leaning towards the A/V receiver as a culprit, although that does not explain the subtitles when you do have sound.
H20-100S x2, HR10-250, DVR40 - 120 hrs, DSR708, R10, R15 (inactive).
DTV & NFLST Since 1997, not looked back yet...