The optical audio output on the TV is capable of outputting 5.1 audio but only when the TV is decoding the audio signal.
* Only available with digital stations/signals that are carrying a 5.1 audio signal.
** Only available with files that have been encoded with a 5.1 audio codec.
*** Only available with apps that carry a 5.1 audio track.
If you are not using a source that has 5.1 audio, you will only receive 2 channel (Stereo) PCM audio.