ffmpeg commands to downmix audio to 2 channels

gistfile1.txt

# LIBOPUS
ffmpeg -i <input> -c:a libopus -b:a <bitrate> <output>

# FORMULA TO DOWNMIX TO 2 CHANNELS AND EMPHASIZE VOICE
ffmpeg -i <input> -c:v copy \
    -ac 2 -af "pan=stereo|FL=FC+0.30*FL+0.30*BL|FR=FC+0.30*FR+0.30*BR" \
    <output>
    
# COMBINED
ffmpeg -i <input> -c:v copy -c:a libopus -b:a <bitrate> \
-ac 2 -af "pan=stereo|FL=FC+0.30*FL+0.30*BL|FR=FC+0.30*FR+0.30*BR" \
<output>


https://superuser.com/a/1410620