How can i get sound from my laptop to my tv using a hdmi cable. the hdmi output is there but dissapears.....?

without the hdmi cable in, it says there is a hdmi out device/option which i cannot use. i then put the cable in and the laptop suddenly forgets what hdmi sound output is. picture comes through alright and i fairly certain the sound card will allow the sound to traverse the hdmi cable, its just being able to select the cable as the output which is the problem... because although the cable is in, there is no option anywhere to select it.