mp3length: 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/mp3length - from package: poc-streamer

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

Error in other languages:
mp3length: bevel nie gevind nie
mp3length: comandă negăsită
mp3length: comando não encontrado
mp3length: commande introuvable
mp3length: command not found
mp3length: käsku ei ole
mp3length: không tìm thấy lệnh
mp3length: komanda nerasta
mp3length: Kommando nicht gefunden.
mp3length: kommandot finns inte
mp3length: komut yok
mp3length: nie znaleziono polecenia
mp3length: níor aimsíodh an t-ordú
mp3length: no se encontró la orden
mp3length: no s'ha trobat l'ordre
mp3length: opdracht niet gevonden
mp3length: parancs nem található
mp3length: perintah tidak ditemukan
mp3length: príkaz nenájdený
mp3length: příkaz nenalezen
mp3length: команда не найдена
mp3length: командата не е открита
mp3length: 命令找不到
mp3length: コマンドが見つかりません