https://ffmpeg.org/ffmpeg.html
https://trac.ffmpeg.org/wiki

# NORMAL copy audio::
> ffmpeg -i input.mp4 -c:v libx264 -profile:v high -level:v 4.1 -c:a copy output.mp4

# NORMAL encode audio::
> ffmpeg -i source.mp4 -c:v libx264 -b:v 650k -profile:v high -level:v 4.1 -c:a aac -b:a 96k target.mp4

# VOLUME LEVEL +2 encode audio::
ok>>> ffmpeg -i source.mp4 -c:v libx264 -b:v 650k -profile:v high -level:v 4.1 -c:a aac -b:a 96k -filter:a "volume=2.0" target.mp4

# 5.1 to 2 channel et/ou [[prologicii]]
> ffmpeg -i source.mp4 -c:v libx264 -b:v 650k -profile:v high -level:v 4.1 -c:a aac -b:a 96k -ac 2 [[-af "aresample=matrix_encoding=dplii"]] -filter:a "volume=2.0" target.mp4

# 5.1 to 2 channel et/ou prologicii
###!/bin/bash
#sudo sysctl vm.drop_caches=3   ##vider le cache mémoire
##OK :: AC3 vers 2 channel ::
#ffmpeg -i XviD.AC3.avi -c:v libx264 -b:v 600k -profile:v high -level:v 4.1 -c:a aac -b:a 96k -ac 2 -filter:a "volume=2.0" BRRip.x264.ac2.mp4
##OK :: AC3 vers 2 channel Prologic ::
#ffmpeg -i XviD.AC3.avi -c:v libx264 -b:v 600k -profile:v high -level:v 4.1 -c:a aac -b:a 96k -ac 2 -af "aresample=matrix_encoding=dplii" -filter:a "volume=2.0" ac2.prologii.mp4

#### CRF#####
ffmpeg -i input.mp4 -c:v libx264 -crf 23 output.mp4
###exemple
ffmpeg.exe -i Sideburns.Sweat.Die.alone.mp4 -c:v libx264 -b:v 650k -profile:v high -level:v 4.1 -c:a aac -b:a 96k target.mp4

EXPLICATION
> ffmpeg -i source.mp4 -c:v         libx264 -b:v          650k    -profile:v high -level:v 4.1 -c:a         aac   -b:a           96k
( ffmpeg -i source.mp4 -codec:video (x264) -bitrate:video (650k)  -profile:v high -level:v 4.1 -codec:audio (aac) -bitrate:audio (96k)
> -ac 2      -af "aresample=matrix_encoding=dplii" -filter:a "volume=2.0" target.mp4
( 2 channel  -audio filter dolby prologic ii       gain +2