Hi @StephenCunningham 

Welcome to the Sonos Community!

I’m sorry to hear your upgrade to Arc hasn’t yet gone perfectly! The good news is that we know there are no hardware faults, so we just need to figure out what the software is doing.

With an HDMI-(e)ARC connection, the devices (and sometimes apps) are able to communicate with each other. One of the reasons for this is so the devices can figure out what the best audio format that can be used is, and use it - I suspect this is where things are going wrong for you.

First of all, are there any movies on Plex that do work? Find a movie that you know doesn’t work and check what audio format the file has, and if there are multiple audio streams, which is being selected by Plex. If, for example, that file has Atmos, please see if you can find a file that has DTS or plain Dolby Digital instead - do they play audio? Please try as many different formats as you can find. If the results are confusing, feel free to post your findings.

If, as a result of the the testing, you find that not all formats fail to play, the best thing to do may be to check if Plex has audio transcoding enabled or not. If not, try transcoding to Dolby Digital in the Plex server’s settings. If all formats fail to play, you may need to disable transcoding, or at least alter the settings.

Please also be aware that the Plex playback app will have it’s own audio settings that are separate from the Plex Server settings.

The TV could also be a part of the issue. Usually, selecting Bitstream or Passthough as the audio output format is best, but Auto will be worth trying too.

Finally, I have never owned an Apple TV, but I have heard of some unusual behaviours as compared to other streaming boxes. Please do try the available options for the output format, but keep an eye on what format the Sonos app reports is playing - whatever it says will be what the Arc receives, regardless of what format is on the file or what format Plex, Apple TV or LG say is playing.

The thing is, all these options add up to a fair amount of permutations of settings. Keeping things on Auto - where that is an option - will probably be the best bet (because of the negotiations performed on HDMI-(e)ARC).

Ideally, someone with Apple TV and the same TV as you will chime in with exact settings - my posting here will put the topic back on the list of those recently active.

If you struggle with anything, feel free to post details and/or screenshots and I’ll do my best to assist.

I hope this helps.

Hi - thanks for your response. I think it must be a setting on the Plex Apple TV App.

When playing files via Apple TV Plex app:

files with Stereo AAC codec or Stereo MP4 play fine
files with 5.1 EAC3 codec play with no sound

I downloaded the Plex App from the LG App Store and playing the files on the LG TV version of the app play without issue.

I downloaded the Infuse app to my Apple TV and played files from my Plex library via Infuse and they play without issue.

So, it isn’t the Arc, isn't the TV but specifically Plex on the Apple TV that I experience this issue.

Hi @StephenCunningham 

It sounds like you have done some good work there! I concur - from what you have told me, it does sound like the Apple TV’s version of Plex is responsible.

I recommend going to Plex support from this point - there should be a forum at https://forums.plex.tv, but I cannot get it to load for some reason. Hopefully, it’s temporary.

For what it’s worth, EAC3 is also known as Dolby Digital +, and is supported by your Arc - you should get sound when playing it!

I hope this helps.