Just bumping this to say:
I still cannot play movies with 7.1 audio from Plex to my Chromecast Ultra.
same same.
Either plex needs to remove/adjust the default chromecast profile that over rides custom xml or google needs to update the firmware.
Same problem…
Just bought a new chromecast ultra to my new lg oled55c9 tv and I can only play 4k with 5.1 audio.
I have an old chromecast ultra connected to another lg tv (mayby 2017 model) and it works just fine with 4k 7.1 audio…
The chomecasts are both upgraded to latest firmware.
Can the problem be that my new tv support 7.1 audio and the old one doesnt?
Same issue here. Waiting/hoping for a fix or xml tweak.
chromecast firmware 1.42.183786 is out.
Seems to be playing all audio again.
Will test more and report back if i remember
My Chromecast ultra is on the same firmware now, yet the issue still persists for me. It is not resolved.
I’m with @agambon here…my Chromecast Ultra is on the same firmware reported by @olly230 (1.42.183786), but I’m getting the same error as everyone else:
Something went wrong - Error code: h4 (Not Supported)
So now HDR isn’t getting detected by my TV anymore…
Plex through Sony Android app is ok, sound works, HDR works. Why not the Ultra?
(before you ask, the CC ultra gets enough bandwidth for full 4k rips)
Yeah, that’s only going to work for a file that happens to have a 5.1 audio stream. That’s not a workaround, it’s just having the file in a format that works instead.
yeah, I just deleted my post. I think it was a lot of information that ultimately is irrelevant to the issue.
TLDR - if you have an alternative audio stream, you can switch to that in the playback settings. Not a fix or workaround for this issue 
i’ve been resorting to using avidemux to manually add a mixed 5.1 stream to my movies, its takes about 5min to do so per movie. is it a pretty solution, no, but it’s all i got till they get their act together
Hi,
Any word o a fix for this? I believe it’s what’s causing my 4k content colouring to appear washed out. The video is direct stream but the audio is transcoding to AC3.
Also impacted.
that’s most likely cause you are trying to play hdr content, there are known issues with that too
This sounds like the issue I’m getting, too (I posted here a short while ago: "h4 (Not Supported)" with 4K x265 HDR video files on Chromecast Ultra).
Is it a hardware limitation?
Has Google responded about the issue?
Have the Plex developers? (there are a lot of paid customers having this issue, so a response would be welcome)
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Basically had to ditch chromecast ultra in favor of the plex for webos because of this. Wish Plex developers could give some insight into this…
I’ve had the same issues for the past several months as well. Seriously disappointed that there’s no reply from anyone at Plex at all so far.
Having this issue for months and it’s extremely irritating.
My temporary workaround? Run an additional instance of Emby to watch the affected media.
I’m serious.
At least it let’s me watch my stuff – it’s not direct playing, but at least I can watch my stuff.
On Plex, I don’t even get the chance to watch a transcoded version of the affected files, which is obviously unacceptable.
It’s frustrating, but my mostly-permanent workaround was to trash my Chromecast in favor of an Nvidia Shield.
The only annoyance is when I visit a friend’s house who runs Chromecast, and can’t play my movies.
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I’ve had this issue for months myself and have been using a couple workarounds.
I have gotten around this by optimizing the movie - I typically choose the custom setting for TV at the highest quality. I believe the audio is transcoded allowing it to play on the Chromecast. Downside is of course you need to wait a few hours.
I’ve had some success in quickly changing the playback quality after starting the video. If I am quick enough, I can do this before it errors out. In this case however, sometimes the quality is very low with the transcoding.
