Did you attempt to run the optical cable from the DCH3416 to the receiver to eliminate a cable box issue. Also, check the av settings on the tv to assure that you are setup properly for using the audio out connection for optical.
I've got a Motorola DCH3416 HD DVR from Comcast and having a heck of a time getting video and audio. I'm running a HDMI cable from the DVR to my Samsung HDTV. I'm then running an optical from the TV to my Sony receiver. I can hear sound from the TV speakers but nothing through the receiver. I've also tried component cables along with audio cables and the same problem.
The above tells me that audio is going to the TV via HDMI but not from the TV to the receiver. The interesting thing is, I had a Series 2 TIVO unit hooked up before the HD DVR...it didn't have HDMI so it was all component cables but the audio worked fine through the receiver. I thought it might be a bad box from Comcast so I'm on my 2nd one now but same problem.