normalize-mp3: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On Debian possible default full paths are:

/usr/bin/normalize-mp3 - from package: normalize-audio

You may install the required package with command apt-get install {package}

Error in other languages:
normalize-mp3: bevel nie gevind nie
normalize-mp3: comandă negăsită
normalize-mp3: comando não encontrado
normalize-mp3: commande introuvable
normalize-mp3: command not found
normalize-mp3: käsku ei ole
normalize-mp3: không tìm thấy lệnh
normalize-mp3: komanda nerasta
normalize-mp3: Kommando nicht gefunden.
normalize-mp3: kommandot finns inte
normalize-mp3: komut yok
normalize-mp3: nie znaleziono polecenia
normalize-mp3: níor aimsíodh an t-ordú
normalize-mp3: no se encontró la orden
normalize-mp3: no s'ha trobat l'ordre
normalize-mp3: opdracht niet gevonden
normalize-mp3: parancs nem található
normalize-mp3: perintah tidak ditemukan
normalize-mp3: príkaz nenájdený
normalize-mp3: příkaz nenalezen
normalize-mp3: команда не найдена
normalize-mp3: командата не е открита
normalize-mp3: 命令找不到
normalize-mp3: コマンドが見つかりません