Help needed, Why does audio delay when editing and remuxing with video?

[bananacreamandpeca]

I have this AVI.
I re-encode this video to 264.MKV (just the video)

Then I demux the audio (with AVIMux) from the original source,
Open up the MP3 in Audition,
Edit the wavefile (keeping the original length, I just do some tonal editing),
re-encode this to MP3,
And then Mux with the MKV-video.

But this always results in the audio out of sync with the video

What don't I understand/What am I doing wrong?

What must I do to get the audio in-sync right the first time?
setting a manual delay is a pain to do,
and I also want to understand why this delay keeps occuring.

[Revgen]

Have you tried demuxing the audio using VirtualDubMod instead?

[bananacreamandpeca]

Yes!

I also demuxed the mp3 with virtualdub (-mod, any diff with just VDub?)

Opened the mp3 in Audition, resaved to MP3 and this will give audio desync.

Why is this?

[manono]

Maybe when originally muxed into the AVI there was a delay which you didn't account for when you remuxed it. Find out and set the delay when remuxing.

I also demuxed the mp3 with virtualdub (-mod, any diff with just VDub?)

Yes, as VDub doesn't support VBR MP3 audio, if it's VBR (did it say something about that when opening the AVI?), then that's a recipe for asynch audio. Use VDubMod and answer "No" to the question. Then demux.