The sound is out of sync
Headphones out of sync with the video, or an external audio file that will not sync.
The sound is behind the video
Wireless headphones, particularly bluetooth, commonly add a fraction of a second of latency which sounds like the video being slightly out of sync. Correct it with Sync in the volume panel, adjusting it while the video plays.
An external audio file will not sync
Use Sync to automatically measure the delay. If it reports that it cannot be measured, try a different reference track: the video's original language usually works best.
If it still cannot be synced, align it with DescribeAlign.
An external audio track starts out fine and drifts
The two recordings run at slightly different speeds, usually one is for PAL format and the other for NTSC. Or they are from different edits of the same video.
Either find audio matching your copy of the video, or align it with DescribeAlign.
A track or subtitle is missing
Tracks with no names, a description not being chosen, or a subtitle or audio file beside the video that is not listed.
Casting from Jellyfin will not work
When TinePlayer does not appear in the Jellyfin app, a server cannot be found, or a cast video is missing a soundtrack.