replace ac3 2ch with ac3 5.1 ch in mkv (and sync)

[umbbra]

hello
I have 2 files: a movie.mkv file with an ac3 soundtrack with 2 channels (384kbps). Same movie but .avi format and ac3 5.1 soundtrack (448kbps).
I extract the 5.1 ac3 and muxed into the mkv file with avimux. Of course, the sound is delayed.
It is possible to sync the soundtrack with the movie (with avimux or something else?) If yes, how can I find the delay and what can I use to sync?
Thanks

[guns1inger]

mkvmerge has a delay option. You can find the delay by opening the file in MPC HC and using the numpad + and - keys to adjust the audio during playback until it syncs. Then use mkvextractGUI to get all the bits of the file out to streams, then use mkvmerge to put it all back together again. You could also try AVI Demux, although I can't vouch for how well it works with MKV files (it does work pretty well for avi files)