I'm having the same problem. This all started after my old multiroom DVR box quit working and I went to Charter to get a replacement. I noticed the problem and took this box back and got a second new box. The problem persisted.
I have the multi-room dvr with the dcx3501-m dvr with Charter service. When turning the channels, the sound on some channels works, and on other channels there is no sound at all. I tried switching hdmi cables and the problem persisted. I switched the hdmi cable from the tv's hdmi input number 1 to the number 2 slot and the problem persisted.
What I have noticed is that when I get to a channel with no sound, I can grab the TV remove and cycle through the available video input choices, AV1, AV2, etc, etc, until I get back to the HDMI2 setting that runs to the cable box. At that point I notice the cable box will display "Conn" as it connects to the cable box. Finally I will hear the sound.
So the problem appears to a connection problem between the cable box and the TV when switching between channels.
So far I have not been able to find a fix other than cycling through the video inputs..which is a major PITA!
BTW, I called Charter 3 times and the could not fix the problem. The next solution for them was to send someone out but that MIGHT result in ME having to pay for the service call if the problem is related to my TV...don't really want to waste that money.
Anyone know anything else?