The sound might be muted. Try pressing the volume up button to restore sound.
Check if it is the same with the other channels. Do a channel search and if there is only one channel with no sound, there is a possibility that the signal strength of the network is poor, or it may be having technical issues.
If you are using DVI or Y, PB, PR, remember to also connect the device's left and right audio output jacks to the TV's L and R AUDIO INPUT jacks.
If the TV is connected to a different device such as a cable or satellite box, DVD, Blu-Ray, etc. make sure that the cable connection is correct (red and white cables for audio). Check if the device has a volume controller and try adjusting it.
Change the TV sound type to “mono” or “stereo”. Refer to the instruction book on how to change the sound type.