What i did was i found the spot where the gap appeared (4 minutes from the end of movie), split the file with Virtual Dub right there, delayed audio for the required time (5760 ms) in the second part and resaved it again. Then i joined the files with Virtual Dub again. The problem is that the last few seconds of first part are now running as in fast forward until the point where the file was split previously - i guess that the video is running faster to "catch up" with the audio (which was delayed in the second part) so that there is no "empty" audio gap. I hope this makes sense. Anyone knows a better way to tackle this?