gnome-volume-control: 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 CentOS possible default full paths are:

/usr/bin/gnome-volume-control - from package: gnome-media

You may install the required package with command yum install {package}

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