mpc123: 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/mpc123 - from package: mpc123

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

Error in other languages:
mpc123: bevel nie gevind nie
mpc123: comandă negăsită
mpc123: comando não encontrado
mpc123: commande introuvable
mpc123: command not found
mpc123: käsku ei ole
mpc123: không tìm thấy lệnh
mpc123: komanda nerasta
mpc123: Kommando nicht gefunden.
mpc123: kommandot finns inte
mpc123: komut yok
mpc123: nie znaleziono polecenia
mpc123: níor aimsíodh an t-ordú
mpc123: no se encontró la orden
mpc123: no s'ha trobat l'ordre
mpc123: opdracht niet gevonden
mpc123: parancs nem található
mpc123: perintah tidak ditemukan
mpc123: príkaz nenájdený
mpc123: příkaz nenalezen
mpc123: команда не найдена
mpc123: командата не е открита
mpc123: 命令找不到
mpc123: コマンドが見つかりません