Mis-recognition of AC3 audio in 2.7

[UKenGB]

I was working on a file yesterday and realised that the audio was mostly missing when playing (pre-viewing) in AVD. I say mostly as there was some background noise and even music, but the main dialogue was missing. When I checked (in MediaInfo), the audio track was actually as follows:-

Audio
ID                                       : 2
Format                                   : AC-3
Format/Info                              : Audio Coding 3
Codec ID                                 : AC-3
Duration                                 : 1h 26mn
Bit rate mode                            : Constant
Bit rate                                 : 384 Kbps
Channel(s)                               : 6 channels
Sampling rate                            : 48.0 KHz
Bit depth                                : 16 bits
Compression mode                         : Lossy
Stream size                              : 238 MiB (14%)
Language                                 : English

But AVD was reporting it as simply 2 channel and presumably dropping the other channels, one or more of which obviously contained the now missing dialog.

I could play this file perfectly in VLC with full audio, i.e. all audio channels output over the stereo system connected to the Mac. Also when using 'Quick Look' on the Mac, all audio played fine. Only AVD seemed to have this problem.

Having edited the file I then saved it and the audio was now 2 track only. Why did AVD downmix the audio. I did experiment with various options while trying to discover why the audio was mostly missing but I thought I had set everything back so it should have saved an exact copy of the original audio track although maybe I got it wrong. Under what circumstances will AVD downmix 6 channel audio to 2 track stereo?

How can I set AVD to simply copy the audio and video without modification (apart from cutting out unwanted portions), but when pre-viewing, actually playing all the audio and not simply dropping channels. Is there a bug that means it is not correctly recognising 6 channel audio?

[Jan Gruuthuse]

When you leave audio in copy mode, it should not alter the channels.
You did check the audio tracks (avidemux 2.7 handles up to 4)
Is the audio changed in the newly saved video? AC3 track 5.1 -> AC3 stereo?
Or is it when playing video in Avidemux? 2 different things. If the last, check in Avidemux Menu: Edit:
Preferences: [Audio] following settings:
- Local playback downmixing: [No downmixing]
- Have you got the correct AudioDevice?

ps: Avidemux is not a mediaplayer, and should not be used as such

Avidemux is a simple tool for simple video processing tasks. The keyword here is simple: it does not offer tools like a timeline, multitrack editing, you cannot freely move or splice audio and video clips from various sources. However, Avidemux allows you to do elementary things in a very straightforward way.

source: Avidemux Quickstart