Problem with AAC audio recode on 2.6.13

[eumagga0x2a]

If you have more than one audio track then you have to pick one you'd like to encode unless you use copy and the target container supports multiple audio tracks. If your source mp4 file contains only a single 5.1 channel ac3 audio track alongside video, there is no point in selecting a track

Prior to the aforementioned git commit you had to downmix to stereo as all the AAC encoder in Avidemux supported was 2ch audio. Now it can encode 6ch as well and the chances are good that the channel mapping is correct.