You may have already tried these:
1. Navigate to the inner portal on the MAG box (The black menu with yellow icons). Go to Audio, Set initial Volume to 100, and set audio output mode to Stereo. Exit and reboot device.
2. As simple as it sounds this is very common to be overlooked. Make sure the audio is not muted, also use the volume controls on the MAG box remote as well as the TV remote. Again check the TV remote for muted. If you are using an audio amp or processor please try using plain old HMDI to the TV to make sure its not an issue with any other equipment.
3. Try another HDMI cable. As simple as this sounds, this can often be overlooked as the problem.
4. Navigate to the inner portal on the MAG box (The black menu with yellow icons). Select software update. Follow the onscreen instructions to complete the update. After update completes please check for audio again.
5. If none of the above works for you please try your MAG box at another premises. New TV, New Network connection, new everything. If it still does not work there maybe a fault with your MAG box. In the event of this please contact the supplier of the box and ask about warranty and returns procedure.