Please shut the TV and Direct TV box down. Unplug the Direct TV box from AC power for minutes. During this time, disconnect and reconnect the HDMI cable on both ends, making sure it is secure. Then plug the Direct TV 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.
If you need further assistance, let me know.
Please disconnect the set from AC power, then press and hold the power button on the TV itself (not the remote), for 10 seconds. After another minute, plug the set back in and press power once. Do you still have the same problem?