No sound for our TV connected to Comcast DVR cable box

For the last few days our TV has no sound. The TV is cable connected via a comcast DVR box. I wonder if I need to exchange the DVR box. The TV sound works for other HDMI ports such as for Roku and DVD player. Thus far restarting the DVR box has fixed the problem, but it seems to happen more often.

PS: One time we had no sound and the picture scaling was too big so I couldn't see the bottom of the screen where you can navigate to Settings / Help to restart the DVR box.