replaygain: 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/replaygain - from package: python-rgain

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

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