mute: 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/mute - from package: aumix-common

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

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