{{{sh
ffmpeg -i <input> \
    -c:v libx264 -map 0:v \
    -map 0:a:0? -c:a:0 aac \
    -map 0:a:0? -c:a:0 aac \
    <output>
}}}


Right idea, but wrong:w

{{{sh
ffmpeg -i 'Angry Video Game Nerd - The Movie.mkv' \
    -c:0 libx264
    -map 0:v -map 0:3? -c:a:0 aac -map 0:4 -c:a:2 aac -map 0:6? -c:s:3 copy \
    avgn-final.mp4
}}}