I just experimented with my MT4G. The head phone jack has some sort of cutout that causes the speakers to not play when a plug is inserted into the jack. If I slowly push in and pull out the head phone plug I can find spots where nothing can be heard from either the head phones or the speakers. In theory, if the plug is fully inserted you should hear sounds through the head phones. If you try slowly pushing in and pulling out the plug and can hear sounds in some positions, I would guess there is a problem with the jack. Hope this helps.