muted: 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 CentOS possible default full paths are:

/usr/sbin/muted - from package: asterisk

You may install the required package with command yum install {package}

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