mate-volume-control-applet: 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/mate-volume-control-applet - from package: mate-media-pulse

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

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