Audio and video sync issue in a 720p mkv file

try like this.

open your video with vlc.
click on "tools"
click on "track synchronization"
under "audio track synchronization"
increase the value using the arrows positively to slow down.
negatively to speed up.
adjust your audio until you achieve synchronization.
move forward in the video to see if it is correct everywhere. note this value
be careful, 0.400 for example corresponds to 400 ms

open mkv merge
drag your video into source file.
click on the audio (it turns blue)
click on "format-specific options"
under "offset (in ms)" enter the value you set in vlc.
if negative, put a minus in front.
under target file indicate the destination folder.
finally click on "start multiplexing"
to try then
slts

Hi
if the offset is constant, note the value with VLC, then report it in mkv merge.
cheers

Could you explain the procedures to me precisely, thank you.