mplayer: 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/mplayer - from package: mplayer2

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

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