Motorola DVR headphone problem.

I recently purchased an Hitachi HDTV, then changed my cable service to include the Motorola (6412, I believe) DVR. I used the TV for a few days - connected directly to cable - while I waited for the DVR to be installed. My headphones worked fine. Once the cable folks installed the DVR my headphones went mute. I hadn't changed the headphone connection. Mediacom says it is not their problem, but it is a very big problem for me. I am nearing 70 and have only about 20% of normal hearing. It is almost impossible for me to understand TV audio without headphones.

I would be very grateful for any suggestions regarding connections or settings that will remedy this situation.

Thank You, Chris Milton