Power dvd-dolbyheadphone not working



whenever i play dvd with my asus dvd player tru’ power dvd, i get just a flat stereo sound with the dolby headphone mode. but when i play anything within my hardisk, it’s working. what did i do wrong? do i need a certain soundcard or a special cable attached to my dvd player?
what should i do to make dolbyheadphone works with my dvd?