To my knowledge, when you are not set to PassThrough, the audio is decoded by the Plex Media Player/App and presented in a format for the audio setup that is detected. PassThrough does NOT decode the audio stream and just passes it through to the next device in the audio system (for me a Pioneer 9ch receiver). You can pass through audio to the TOS-link optical output or to the HDMI port, but TOSlink can only handle DD or DTS, no Dolby TreuHD or DTS-MA.
Now I am making a lot of assumptions here; but, I am guessing that when you are NOT set for passthrough the PMP is seeing just a set of speakers and downmixing to stereo. When you are set to passthrough the next device inline, which I am guessing is an audio decoder chip built into your TV, gets the un-decoded audio stream and decodes it properly. Now I am not personally familiar with Sony’s acoustic surface audio system but it seems that that system has the full decoding system in it just like my receiver. So I would leave PMP set to passthrough, be it Optical or HDMI Which ever works better, and let the TVs decoder handle all the audio decoding work. There will be less load on PMP w/o having to decode the audio and the Sony chip will most likely provide better sound quality.
