Possible Cause

1. Improperly connected source signal cable.
2. Volume is turned all the way down.
3. [MUTE] is turned on.
4. No external speaker connected. 

Remedy

1. Make sure that both video inputs and sound inputs are correctly connected.
2. Press [VOL UP] or [VOL DOWN] button to hear sound.
3. Switch MUTE off by using the [MUTE] button.
4. Connect external speakers and adjust the volume to a suitable level.