Hi,
is it possible, that the DVD have Dolby Digital 5.1 and the Player want to 'throw' the sound to a optical/coaxial output?
Check handbook and settings on which output the player want to play the sound.
Not all players automatically remix the sound for stereo/dolby surround output. sometimes you have to choose the sound in the dvd menu (stereo/dolbysurround 2.1)
and sometimes you have to 'motivate' the player to do it.
Post model of the device, maybe someone can help with setup.