Audio sync problem on mkv movie

I want to know how to fix this. Its a 700meg movie.
The audio sync error doesnt come into effect until 3/4 way through the
movie, and then suddenly 2secs or something after they moved their lips, the words are audible.

Can I rectify this in virtualdub ? and how do I do this ?

The format is mkv so I might have to convert into avi or mpg first.

 

VirtualDubMod will open and 'save as' .mkv

Here's the unofficial VDub advanced forum.
http://forums.virtualdub.org/index.php?s=5b31915137ffa3ceec5f21ef7cd85e65&act=SF&f=5