There can be several reasons for no sound.
1. The volume on the box is reduced - You need to press the Volume up button on the remote control
2. Not good connection of HDMI to the TV - Try to remove the cable from both ends and plug it back in. Make sure its plugs all the way into the socket from both ends.
If it didnt help try to replace the cable.
3. On some TV sets when another device is connected to the TV its taking over the Audio input - Try to disconnect all other connected devices and leave only the box connected to the TV.