Substituting the audio track in a mkv file: the correct calculations

[buongiorgio]

Hi folks,
I have a problem that is driving me crazy.

I have two files of the same movie:
1. Avi file, total dimension 1.36GB, 25fps, language Italian;
2. Mkv file, total dimension 5GB, 23.976fps language English.

I extracted the ac3 audio track from the AVI file and would like to add it to the mkv file. I do not like to manually tune the delay, so i tried to do some calculations based on the difference in frames. Using virtualdub I found out that the "gap" between the two files is 14 frames (e.g. the same scene in the Avi file is at frame 869 and in the Mkv at the frame 855). So, according to my knowledge, to obtain a perfectly synchronised movie I have to do the following:
1. Extract the audio track from Avi with virtualdub;
2. Cut off 560 ms from the audio track (eac3to original.ac3 cut.ac3 -560ms) the 560ms is 14/25 i.e. the gap in frames divided by the fps;
3. "Slowing" the audio track (eac3to cut.ac3 final.ac3 -slowdown);
4. Muxing the audio and the mkv together.

I did it and the audio is significantly out of sync.
What did I wrong?
Thanks,
G.

[AlanHK]

the "gap" between the two files is 14 frames (e.g. the same scene in the Avi file is at frame 869 and in the Mkv at the frame 855).

Does say,
frame 10014 match frame 10000?
20014 match 20000?

Otherwise, you might match the audio tracks in Audacity.
You can look for noises like doors closing in both tracks, then just slide the Italian track to sync one point near the start, then stretch it using "Change tempo" effect to sync another point near the end.
Then mute the English track, save in the format of choice.

[buongiorgio]

the "gap" between the two files is 14 frames (e.g. the same scene in the Avi file is at frame 869 and in the Mkv at the frame 855).

Does say,
frame 10014 match frame 10000?
20014 match 20000?

Yep, this 14 frames gap is constant throughout the whole file.

Otherwise, you might match the audio tracks in Audacity.
You can look for noises like doors closing in both tracks, then just slide the Italian track to sync one point near the start, then stretch it using "Change tempo" effect to sync another point near the end.
Then mute the English track, save in the format of choice.

I would prefer to avoid this kind of editing, it is too much complex for my skills.