I have a file, TestAC3.avi, designed to test Dolby 5.1 on all 6 speakers. This file plays back correctly using all versions of VLC media player. With Avidemux, all front speakers playback correctly, however the rear left is played on the front right, and the rear right is not played at all.
The sound output is via the computers Digital Output (S/PDIF) to a Yamaha RX-V473 system.
The file can be obtained from: http://www.tfm.ro/win32-projects/test-avi-ac3/
Avidemux is not a video player. It all depends on how you did setup avidemux and what the primary track is of your video.
Main Menu -> Edit -> Preferences -> tab [Audio] -> and the selection in:
Local playback downmixing: [no downmixing]
AudioDevice: [PulseAudioS]
another issue that comes in to play: Channel Arrangement for Multi Channel 5.1 Audio Formats (http://www.avidemux.org/smf/index.php?topic=10957.msg58738#msg58738)