This can be caused by one of two things.
First, let's try the following.
First, make sure the Volume is turned up on BOTH the Cable Box AND the TV. Each has its own independent volume control
and if one or the other is turned down, then you will have no sound.
Beyond this, please reset the system by following the steps below:
Please shut the TV and cable box down. Unplug the Cable box from AC power for one minute. During this time, disconnect and reconnect the HDMI
cable on both ends, making sure it is secure. Then plug the cable box back in, turn on and let it reboot. Once it's rebooted, then turn on the TV and attempt to watch a program.
The sound should be back on at this point.