Normalizar o volume de áudio no linux via linha de comando

mp3gain -c -r *.mp3
ou
sox --norm infile outfile
ou

mp3:
find /home/$USER/Music/ -type d | while read FOLDER; do cd "$FOLDER" && mp3gain -a -k *.mp3; done

m4a:
find /home/$USER/Music/ -type d | while read FOLDER; do cd "$FOLDER" && aacgain -a -k *.m4a; done

FLACs

find /home/$USER/Music/ -type d | while read FOLDER; do cd "$FOLDER" && metaflac --add-replay-gain *.flac; done

#!/bin/bash

set -e
set -u

FILE="$1"
DIR=`dirname "$FILE"`

FILENAME=`basename "$FILE"`
WAV_FILENAME="${FILENAME}.wav"

LOUD_DIR="$DIR/loud"
WAV_FILE="$LOUD_DIR/$WAV_FILENAME"
LOUD_FILE="$LOUD_DIR/$FILENAME"

mkdir -p "$LOUD_DIR"

avconv -loglevel quiet -i "$FILE" "$WAV_FILE"
normalize-audio -q -a 1 "$WAV_FILE"
avconv -loglevel quiet -i "$WAV_FILE" -c:a libvorbis -b:a 50k "$LOUD_FILE"
rm "$WAV_FILE"