ffmpeg -i video.mp4 -vn \
-acodec libmp3lame -ac 2 -ab 160k -ar 48000 \
audio.mp3
ffmpeg -i video.mp4 -vn \
-acodec libmp3lame -ac 2 -qscale:a 4 -ar 48000 \
audio.mp3
The VBR example has a target bitrate of 165 Kbit/s with a bitrate range of 140...185.
ffmpeg -i input.file -map 0:a:0 -b:a 96k output.mp3