[MPlayer-users] ASS subtitles in an MKV aren't synced or timed correctly

[Abu Zaher]

it might be a bug but 2 solve the issue try converting the subtitle
into another subtitle format, preferably into mplayer's mpsub format:

mplayer your_file.avi -sub source_subtitle.ass -dumpmpsub

then play the file with the new subtile:

mplayer your_file.avi -sub dump.mpsub

still the same problem?

Then use another subtitle conversion tool, there are plenty of those.