Almost always this is a player issue as mentioned. I’ve had similar issues with some files where audio just doesn’t work on one player but works fine for another. Same with subtitles. Set to an external player like VLC and see if that fixes it. (For what it’s worth, I mostly access Jellyfin from my Chromecast and I have it set to prompt me for a player each time I start watching something, one of them always works but for my files it isn’t consistent which)
I found what really helps Jellyfin on my Chromecast is setting the player manually. There’s a setting to make it ask which player to use when starting a show and if one doesn’t work, 99% of the time the other one works fine.
Sometimes switching players doesn’t fix subtitles for me, in those situations I usually have to toggle subs a few times or restart the stream and they actually work.
In my opinion it’s a minor enough inconvenience given Jellyfin is 100% free and open source, whereas Plex is tracking you and charging you. But of course maybe your media is in some more difficult format than mine.
Hai, Kazuma desu