Sync audio and video automatically

I don't have a solution, but if it is in sync in the beginning and not at the end, I think the only two issues could be frame rate or you have a different version of the movie with more or less content.

There is no "super-easy" way, but if you are only watching it once, sync the first half to watch and then re-sync the audio to the second part, skip to that part and continue watching.

If you are wanting a "perfect" video file, you will need to use a video editor easier for a more precise audio cut or an audio editing program to edit the audio stream to match the video. I use mkvtoolnix + mkvcleaver to rip the video or audio out for editing, the video in the mkv container is usually .mp4 with an aac audio file. After you've edited the audio to match the video, save the audio file and then use mkvtoolnix to re-package it in the mkv container in a minute - it's possible to keep the old audio file but add the new Spanish stream as the default audio track( like dual audio channel videos). I used to use this program often to rip/add subtitles, sometimes audio.

This reminds me of subtitles that are literally one or two seconds longer/shorter, but somehow magically don't work with your version of the movie. You can fix it by editing the subtitle or increasing/decreasing the delay, but it starts to get bad again and it gets worse and worse as the movie goes on. How is that even possible? It's not like the scenes in the movie are actually different length from version to version. So why does it keep getting out of sync again? It's dark magic, I tell you.

A video editor should help, so you can split the audio when it starts to get out of sync and add delays, speed up or adjust portions to your liking. Then save / export when you're done

My guess is that the mkv file is NTSC (or the source was) and the DVD is PAL. That's why they're different in length, which makes the audio getting out of sync progressively. PAL films become 4% shorter due to the conversion from 24fps to 25fps. NTSC uses a method called 3:2 pull down telecine to convert from 24 to 30 (29.97 actually) which doesn't change the length.

So you have to modify the audio to make it 4% (edit: 4.166% would be the correct value) longer. That process will change the pitch of the audio, but there's software that does it maintaining the pitch.

What is the movie called?

And if you got a dvd shouldn't that have english with spanish subtitles already?

I think its a lot easier to find a different version then trying to figure this out.

With virtualdub you are able to change the rate of the audio dub to fit the format of the video.
You only need to save the video with the new audio dub afterwards.

However it is almost a decade ago since i last was into movie editing so i can´t guide you trough anymore.

http://virtualdub.org/