kmixctrl: 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/kmixctrl - from package: kdemultimedia

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

On Debian possible default full paths are:

/usr/bin/kmixctrl - from package: kmix

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

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