Convert the MP4 audio to 2-channel from surround using FFMPEG

Convert the MP4 audio to 2-channel from surround.

ffmpeg \
    -i INPUT \
    -c:v copy \
    -af "pan=stereo|FL < 1.0*FL + 0.707*FC + 0.707*BL|FR < 1.0*FR + 0.707*FC + 0.707*BR" \
    -vol 512 \
    -movflags \
    +faststart \
    OUTPUT \
;