How to demux AC-3 audio stream from *.mkv video?

[pxstein]

I have a big video file (>8 GB) with an MPEG-4p10/AVC/h.264 encoded video part and an AC-3 encoded audio part.
Both tracks are embedded in a *.mkv container.

Since I need not such a high resolution and prefer smaller video files on my hard disk I want to re-encode it with Avidemux to a lower bitarate and MP3 as audio codec.

Whenever I do this the resulting video is asynchronous.

So sometimes the audio part is approx 9 seconds ahead, other only 4 seconds.

Since I encoded with the same settings 50 other videos the used current Avidemux are correct.

So I am searching for another way to produce a smaller video file.

How can I achieve this?

How do I de-mux the audio part with mkvmerge GUI from mkvtoolnix package?

[Baldrick]

Have you tried gMKVExtractGUI or mkvcleaver? Then mux back with mkvmergegui (part of mkvtoolnix).

[sneaker]

If it becomes async it may be variable framerate (incl. irregularities like audio gaps). VidCoder and HandBrake seem to be able to handle those.