Audio sync points - is there such a thing?

[Shasoosh]

Here's my problem. I'm starting a project where I want to sync an audio stream from a.avi (mp3 or ac3) to a video file, b.mkv for that matter. the video content is identical (credits aside)
a.avi is 25FPS
b.mkv is 23.976FPS

This is what is giving me the best result yet:
I'm converting a.avi to a.mp3 with 23.976 frames using meGUI. (I suspect that meGUI isn't giving me perfect results but I'm not sure)
Then I'm using MKVMerge to mux those two and maybe giving it a bit of delay as needed. The problem is that it's not perfect. I don't know what causing it but even if the sync is 100% accurate at the beginning of the movie, somewhere along the middle the audio will be half a second - 1 second out of sync.
I know that with subtitles you can set syncing points, is there something like that for audio?
Can you think of anything else that might help me out?

Thanks

[jagabo]

I know that with subtitles you can set syncing points, is there something like that for audio?

No. If you did the slowdown properly the new audio would sync. Unless the two sources were not the same cut of the movie -- which is often the case with international releases.

Import the audio into an audio editor (Audacity with the ffmpeg source plugin can read it directly from the source video). Change the duration by a factor of 1.04271, ie (25 * 1001 / 24000). Save. Mux.

[AlanHK]

I know that with subtitles you can set syncing points, is there something like that for audio?
Can you think of anything else that might help me out?

Thanks

In Audacity, open the original audio from the target video (Audacity can probably open it directly from the video.)

The open the new audio as a new track. You can compare the waveforms.
The "sync points" are ideally sharp noises. Like doors slamming, cups hitting on a table, etc.
Use the "Time shift tool" to slide the new track along so such a sync point near the beginning lines up with the original track.
Zoom in to make it perfect.
Write down the exact time of this point down to the millisecond

Zoom out and go to the end. Find a similar sync point.

Now in the original track use the select tool and select the segment between the two points.
Say 1000 seconds. You will see the length of the selection in an info box at the bottom.

In the new track, select the segment between the corresponding points.

Now go to "Effect/Change tempo"
You'll see "Length (seconds)" Put the length of the original track segment here.
Now you will see a "Percent change". Say 5%.
Copy this number. Cancel "Change tempo" -- we only want this number.

Now select the entire new track. Do change tempo again, paste in the percentage.
Click "OK".

When it's done, you may have to slide the first sync point a bit to resync, now both sync points should align.
Check if the sound in between is also correct -- if not, you have different edits and will have to sync scene by scene.