Here is the relevant information from the product manual:

"When the Home Theater receiver is set to On, you can hear sound output from the TV’s Optical jack. When the TV is
displaying a DTV(air) signal, the TV will send out 5.1 channel sound to the Home theater receiver. When the source is a
digital component such as a DVD / Blu-ray player / Cable Box / Satellite receiver (Set-Top Box) and is connected to the
TV via HDMI, only 2 channel sound will be heard from the Home Theater receiver. If you want to hear 5.1 channel audio,
connect the digital audio out jack on DVD / Blu-ray player / Cable Box / Satellite receiver (Set-Top Box) directly to an
Amplifier or Home Theater, not the TV."