I’m lucky to have an LG C9 with eARC. I found running Kodi with Plex for Kodi addon works. You can get DTS to transcode to 5.1.
Setup the audio in advanced audio settings in Kodi first. Not ideal but until Sonos or Plex come up with something it’ll do for now.
I’m unable to get the Plex for Kodi Add-on to transcode DTS to DD 5.1 on the Sonos ARC. What was your advanced audio settings in Kodi?
Change the settings in Kodi - ensure audio channels is set to 2.0 and then enable transcoding to AC3 lower down. Untick all DTS options.
I can take a photo of my settings if that doesn’t work 
Awesome…thanks for the tip!
Got it to work! Thanks Again!
An important detail I overlooked is that “Enable Dolby Digital (AC3) transcoding” is only accessible via the Expert mode option in Settings / System.
I was just about to say that haha. Glad it’s working.
Does anybody have any workarounds for this? I’m now unable to watch DTS files in Plex, I have to switch to Kodi and it’s pretty annoying.
Unfortunately we haven’t been able to find any. At this point we’ll need a client update from the Plex team to request the correct transcoding from the server.
@IanDBird / @sixones do either of you have any thoughts on this?
Dang. I used to be able to turn off passthrough for DTS files but now that doesn’t work with the multi-channel PCM changes…
Not Sure if I should start a new thread… And i’m not at Sound Savy as you guys…
I have a Panasonic OLED TV and can’t get PLEX to switch to anything but 2.0 Audio… (when plugged in ARC mode)
I tried directly on the TV (pan app) , ChromeCast and Amazon Fire. Both ChromeCast and Amazon will work when plugged directly on the TV but not through ARC. I Must add that the rest of the apps in the devices work fine (Disney+, NetFlix, PrimeTV, etc etc…) i know its the Plex APP but Why and what must i change to get this working ! 
(my amp does not do 4K passthrough)
I think your issue might be unrelated. The issues described here are only for the Sonos Arc (not to be confused with HDMI Arc), and only for DTS content.
I did more tests on my side. Seems the problem is also in regards to DTS. I beleive the TV cannot pass DTA through ARC and its lost. (or converted down to 2.0) If i read 5.1 content in plex it works. I guess NETFlix and Amazon Prime either auto convert to 5.1 or don’t have DTS content… is there a way to force Plex to Transcode to 5.1 (if the audio included in file is only DTS)
Must steaming services use Dolby, which is likely why you’re not running into it there.
There isn’t a way to get Plex to transcode DTS audio. That’s basically what we’re asking for here. The Nvidia Shield already knows that DTS isn’t supported but Plex isn’t doing anything with that information and thus isn’t asking for transcoding. Our request here is to get Plex to implement that.
I am having the same situation here. My system includes PMS on Synology DS214 and LG TV 60UH617T. I installed both Plex and Xplay on my TV and want Plex to transcode DTS to Dolby Digital (AC3) to feed my Sonos Amp but has not been able to do so. I have read that you can edit the XML profile to tell PMS to transcode DTS to AC3 but could not find the relevant XML file to do so.
I have a Mibox S with Kodi installed on it, which can solve the problem, but I don’t want to add another equipment to my currently minimalist system.
Any help would be much appreciated.
Editing the profile doesn’t work anymore anyway. Since the client is explicitly telling the server what it supports when it requests the video, the server prefers that over any profile it has. The bug, as described in this thread, is that the Plex client is requesting DTS content despite the Android TV OS being manually or automatically configured to not support it.
The only solution (which uses the normal plex client) I have with my LG C8 is to switch the audio passthrough to PCM mode whenever I play DTS content. However, this has its own set of issues for LG C9 folks.
Yes, that is also my temporary solution: change the HDMI Arc output to PCM instead of auto. I suppose that means the signal delivered to my Sonos Amp will be stereo, right?
Correct, it’ll be stereo (2.1). Another workaround I found is to go to the quality menu and ask it to “convert automatically”. At least in my experience this has led Plex to transcode the audio to AC3, but it does also unfortunately transcode the video.
Unfortunately that option will not work for me, since my Synology DS214 is not powerful enough to transcode video.
Today I try my Mibox S. The fun part is that with Plex on Android TV, there is this Passthrough setting, where I select Optical, which pops up another option enabling AC3 transcoding.
Yup, that might work for your use-case. However, Plex will actually believe you’re using optical so you’ll lose DD+/TrueHD.
