Am schnellsten geht es mit map_channel
und pan
für den Downmix:
ffmpeg -i in.mov -filter_complex "[0]pan=stereo|FL=FC+0.30*FL+0.30*BL|FR=FC+0.30*FR+0.30*BR[st]" -map 0:v -c:v copy -map 0:a -map_channel 0.1.0:0.1 -map 0:a -map_channel 0.1.1:0.2 -map 0:a -map_channel 0.1.2:0.3 -map 0:a -map_channel 0.1.3:0.4 -map 0:a -map_channel 0.1.4:0.5 -map 0:a -map_channel 0.1.5:0.6 -map "[st]" output.mov