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In Copy mode, Avidemux will just copy the audio source (external, internal, etc) to the output file. The only processing that works in copy mode is audio shift to correct for desynced audio.
These filters must be used with a manual video framerate change as the filters change the synchronisation.
An option to dynamics increase or decrease the overall volume of the audio. Check the Gain enable this option.
The mixer will change the number of channels that you hear. The most common usage is to go from 5.1 sound to stereo. You will probably want Dolby Pro Logic II downmixing. Beware that to be efficient, Dolby Pro Logic audio must not be encoded with a low bitrate (~ < 160 kbits).