I have a problem

[TheDreadPirate]

Jellyfin/ffmpeg does not support HWA on RPi4's. Not that the RPi4 has any useful media engines anyway.

Many Jellyfin clients support direct playing the first audio and subtitle track (SRT/ASS subtitles only). If you select a secondary audio track it will require a remux on most clients, which your RPi should be able to handle. But if your subtitles are PGS, or some other image based subtitle, that would require burning in the sub. Which requires transcoding.

You either need hardware capable of handling transcoding or remove all the audio tracks you don't want so that there is only the one. And have SRT subtitles.