No sound

[bitmap]

First, just as an explainer, MKV is a container and says nothing about the video or audio streams it contains. They can be pretty much any video/audio codec out there, plus a number of other factors I won't go into. Just know that MKV tells you very little about a file.

Second, what client are you using for playback? And how did you set up your Jellyfin server (e.g., install script, docker)? What is your host computer's OS?

Lastly, logs would probably help. You can easily find them through Dashboard > Logs. Please post them, both Jellyfin and ffmpeg logs, to a site like privatebin.net.