I've also been experciencing this problem over the last two days on my LG C4, were I'm only getting stereo sound when playing my Dolby Vision + Atmos mkv files using Jellyfin, while HDR10 files play fine with surround sound.
After troubleshooting this further today it seems that the remuxing to mp4 is actually causing the issue, if I direct play the files in another way they play just fine, I get the Dolby Vision logo showing up and the surround sound works using my old home theater setup.
After trying way to much other stuff I tried putting the file on a usb stick and the built in mediaplayer of the tv which played the mkv file just fine, the Dolby Vision logo showed up and surround sound worked, to my surprise.
For now I've installed the DLNA plugin on my Jellyfin server so that I can Direct Play the files without Jellyfin trying to remux them, using DLNA should stream the file as is, this works without a problem on my end both DV and surround sound worked.
To install the plugin you open the server dashboard and click on "Catalog" in the left panel, there you should find the DLNA plugin, click on it and install it.
After the plugin has been installed, you need to restart the server to activate the plugin. Then you open the input selection on your tv and you should see a new option with the name of your Jellyfin server to the right of the HDMI inputs (IIRC).
During my troubleshooting I've seen it mentioned quite a few times that LG tv's/WebOS is unable to play DV mkv files. This is not what I observed so maybe this was recently fixed or is unique to newer models or something, I do not know I've only had the tv for a few weeks and I haven't come across useful changelogs for WebOS either.
Furthermore I would like to add that I've not experienced problems with the "Prefer fMP4-HLS Media Container" option checked playing my SDR and HDR10 HEVC (and maybe other) files, having this option checked only was a problem when trying to play the DV mkv files that Jellyfin was still trying to remux.
So perhaps actually Direct streaming these files as the description of this option suggests would also (hopefully) fix that.
Lastly I've also had intermittent stutters/buffering, skipping or the playback stopping completely when I actually tried to watch the remux that Jellyfin is producing, this does not seem to happen when Direct Playing the files, but I'm not yet sure of this as I haven't watched the Direct play long enough.
I hope this helps other people, and would appreciate it if other people could confirm that Direct playing Dolby Vision mkv files works on their end using the above (or other) methods. Should hopefully be easier to get it fixed then
After troubleshooting this further today it seems that the remuxing to mp4 is actually causing the issue, if I direct play the files in another way they play just fine, I get the Dolby Vision logo showing up and the surround sound works using my old home theater setup.
After trying way to much other stuff I tried putting the file on a usb stick and the built in mediaplayer of the tv which played the mkv file just fine, the Dolby Vision logo showed up and surround sound worked, to my surprise.
For now I've installed the DLNA plugin on my Jellyfin server so that I can Direct Play the files without Jellyfin trying to remux them, using DLNA should stream the file as is, this works without a problem on my end both DV and surround sound worked.
To install the plugin you open the server dashboard and click on "Catalog" in the left panel, there you should find the DLNA plugin, click on it and install it.
After the plugin has been installed, you need to restart the server to activate the plugin. Then you open the input selection on your tv and you should see a new option with the name of your Jellyfin server to the right of the HDMI inputs (IIRC).
During my troubleshooting I've seen it mentioned quite a few times that LG tv's/WebOS is unable to play DV mkv files. This is not what I observed so maybe this was recently fixed or is unique to newer models or something, I do not know I've only had the tv for a few weeks and I haven't come across useful changelogs for WebOS either.
Furthermore I would like to add that I've not experienced problems with the "Prefer fMP4-HLS Media Container" option checked playing my SDR and HDR10 HEVC (and maybe other) files, having this option checked only was a problem when trying to play the DV mkv files that Jellyfin was still trying to remux.
So perhaps actually Direct streaming these files as the description of this option suggests would also (hopefully) fix that.
Lastly I've also had intermittent stutters/buffering, skipping or the playback stopping completely when I actually tried to watch the remux that Jellyfin is producing, this does not seem to happen when Direct Playing the files, but I'm not yet sure of this as I haven't watched the Direct play long enough.
I hope this helps other people, and would appreciate it if other people could confirm that Direct playing Dolby Vision mkv files works on their end using the above (or other) methods. Should hopefully be easier to get it fixed then