mpgdemux: command not found

Have You tried accessing the binary with the full path?

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/mpgdemux - from package: mpgtx

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