Mixer.app: 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/Mixer.app - from package: mixer.app

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

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