I have not found a solution to my problem here (and also checked Uncle G.).
I added srt subtitles to an mp4 file using the VLC player ("Add subtitle file ..."), but these appear late. Then successfully adjusted with "Track synchronization" (under Tools).
Now I would like to save this change permanently so that when the video is called up repeatedly from the hard disk, the subtitles (as corrected before) appear exactly in time. I cannot believe that this is only possible with other video software (e.g. Handbrake or AnyVideo) that I do not master
It would be great if someone could help me!