I am writing this post in the hope it may help those struggling with the above issue(s). The first topic which appears on this in Google is the post at https://en.community.sonos.com/home-theater-228993/solving-the-51-dts-issues-with-the-playbar-34523, which is now out of date. For my part I wanted to set out what I did to "resolve" this issue (or more accurately, work around it).
Since the above post, Sonos have written a few useful notes. The most helpful is: https://sonos.custhelp.com/app/answers/detail/a_id/1945/~/maximizing-playbar-sound-from-your-television.
A further note deals with the XBOX work around (https://sonos.custhelp.com/app/answers/detail/a_id/2490/session/L2F2LzEvdGltZS8xNDQxMjA4NzE0L3NpZC9JdER6cXR2bQ%3D%3D) and another provides the details for PS4 (https://sonos.custhelp.com/app/answers/detail/a_id/3075/related/1).
Playbar with 2 Play:1s and a Sub is a 5.1 Surround system. However, because the Playbar connects through a Digitial Out/Toslink/SPDIF, its limited bandwidth prevents it from EVER being compatible with DTS-HD, Dolby True-HD or any other lossless/HD audio. However, it is possible to play Dolby 5.1 providing your TV can output a Dolby Digital signal through the Digital Out.
People realise that they are only listening to Stereo when they go to About My Sonos System and realise that the AUDIO IN listed under PlayBar is STEREO. When its 5.1 it will say Dolby Digital 5.1, or when Dolby Digital 2.0, exactly that.
DTS will have to be re-coded as Dolby Digital to be played by the PlayBar, else it will be silent (displaying "Audio not supported"). There is no way of playing DTS itself.
Work arounds: My experience
I read the above post and went off to buy a switch (although, this one in the UK http://www.amazon.co.uk/gp/product/B00DZIXIMQ?psc=1&redirect=true&ref_=oh_aui_detailpage_o01_s00, called Portta 4x1 HDMI Switcher with Toslink and Digital Coaxial Port 3D Splitter with Remote and AC Adapter/ 3D 1080P). When I connected my BD-Player to this by HDMI, with my STB and TV, and connected the switch to the Playbar, it started to play 5.1. Result. However, this is a poor solution as you have to manually switch between inputs, have another remote, cannot use a magic remote or the input management system on the TV etc.
I thought about buying an XBOX (which my research indicted is the better media system when compared to PS4) it (with PS4) decodes DTS to DD 5.1 "on the fly.
I also thought about buying the OPPO BDP-103 for a whopping £400 as suggested by the post.
However, there are literally a handful of BD Players which also re-code DTS to DD (please see SONOS's own post). I purchased the Samsung BD-H6500 for a very reasonable sum and can now play DTS easily on the Playbar (obviously, recoded to DD5.1).
I have set my system up as follows. My LG 55EC930V is connected to the Playbar. This TV outputs Dolby Digital (and I think, even Dolby Digital Plus) via Digital Out. I have then connected my Virgin Media STB to the TV via HDMI (STB set to output Dolby Digital Audio) and the BD Player via HDMI, which is set to output recoded DTS.
My issue was that my wired Home Cinema system (Panasonic SC-BTT270) would not output DD as it only had an Optical In, so I had to buy a new player. I highly recommend the one I bought, it plays great.
Also, my Playbar also plays 5.1 from Netflix, Amazon Prime video and Live TV (though, not Youtube for some reason).
Other work arounds
You can also work around the DTS issue by connecting the Playbar to an XBox or PS4, or at worst, a switch.
I hope this helps!!!