Chck you TV manual to see of what it's capable. If that doesn't help, and it didn't look too good to me, you may want to call customer support.
A lot of TV's will only output digital sound if it came from it's own internal ATSC tuner, which is only good for OTA signals. If you're using a cable box or DVD player, it might not work at all or, at best, pass two channel PCM.