mpg321: 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/mpg321 - from package: mpg321

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

Error in other languages:
mpg321: bevel nie gevind nie
mpg321: comandă negăsită
mpg321: comando não encontrado
mpg321: commande introuvable
mpg321: command not found
mpg321: käsku ei ole
mpg321: không tìm thấy lệnh
mpg321: komanda nerasta
mpg321: Kommando nicht gefunden.
mpg321: kommandot finns inte
mpg321: komut yok
mpg321: nie znaleziono polecenia
mpg321: níor aimsíodh an t-ordú
mpg321: no se encontró la orden
mpg321: no s'ha trobat l'ordre
mpg321: opdracht niet gevonden
mpg321: parancs nem található
mpg321: perintah tidak ditemukan
mpg321: príkaz nenájdený
mpg321: příkaz nenalezen
mpg321: команда не найдена
mpg321: командата не е открита
mpg321: 命令找不到
mpg321: コマンドが見つかりません