mpcgain: 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/mpcgain - from package: musepack-tools

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

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