New Samsung Frame - No Sound from Center Channel

I just installed a 65" Samsung frame TV. Both my external devices, Xbox one and Comcast Cable Box are connected to the TV's Connect Box via HDMI. I then connected the Connect Box to my Yamaha surround sound receiver via an optical cable. The issue is, when playing a movie from the Xbox or Cable Box, I don't get dialog from the center channel. I do hear sound from all other speakers such as explosions, gun fire, etc but nothing from the center channel. I do get full sound from the center channel if I play a movie from Netflix via the Smart TV interface. I have changed the audio output on the TV to optical Dolby Digital. Any ideas what the issue could be?