Increase the volume -- and impair the quality -- of your audiobooks

audiobook-volume-amplifier.sh

#!/bin/bash

set -e

if [ $# != 0 ]; then
    echo "Usage: $(basename "$0")

This program accepts no arguments. It takes all the *.mp3 files
in the current directory and makes them louder by first detecting
their mean and max volumes and then amplifying the mean volume by
half of mean or max (whichever is greater). So it clips the samples
that are over the peak possible volume (-0dB) -- i.e. some
information lost.

Works well enough with audio books. So you can enjoy them even while
commuting in heavy traffic.

It optionally creates an m3u playlist."
    exit 0
fi

for prog in ffmpeg awk; do
    if ! command -v $prog >/dev/null; then
        echo "ERROR: missing command '$prog'. Install package '$prog'."
        exit 1
    fi
done

if [ ! "$(ls ./*.mp3 2>/dev/null)" ]; then
    echo "No .mp3 files in current directory. Nothing to do."
    exit 2
fi

OUT="$(pwd)/louder"
mkdir -p "$OUT"

ffmpeg="ffmpeg -hide_banner -nostats"

for file in *.mp3; do
    echo "Processing file: '$file'"
    echo -n ".. detecting mean volume "
    read mean_volume max_volume < <(
        $ffmpeg -i "$file" -af volumedetect -f null - |& 
            awk -F'[: ]' '/_volumedetect_.*(mean|max)_volume/{ print $6 }' |
            xargs)
    echo "(mean: ${mean_volume}dB, max: ${max_volume}dB)"

    volume_change=$(awk "BEGIN { print ($mean_volume/2 < $max_volume) ? -($mean_volume/2) : -($max_volume) }")
    echo ".. increasing volume by ${volume_change} dB"
    $ffmpeg -loglevel error -y -i "$file" -af "volume=${volume_change}dB" -b:a 64k "$OUT/$file"
done

echo "Louder files written to ./$OUT/"
echo
echo "Do you want to create a playlist?"
echo "Type the playlist name to create an m3u playlist, otherwise just pres Return."
echo
read -p "Playlist name []: " playlist
if [ ! "$playlist" ]; then
    exit 0
fi

playlist="$(echo "$playlist" | sed -E 's/[^[:alnum:]]/_/g')"

mkdir -p "$OUT/$playlist"
mv "$OUT"/*.mp3 "$OUT/$playlist"
(cd "$OUT";
 ls "$playlist"/*.mp3 > "$playlist.m3u" )

echo "Done."
echo
echo "Playlist created as: '$OUT/$playlist.m3u'"
echo "Files are in: '$OUT/$playlist/'"
echo "(You need to copy both.)"
echo
echo "Good day!"