jack_simple_client: 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/jack_simple_client - from package: jack-audio-connection-kit-example-clients

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

On Debian possible default full paths are:

/usr/bin/jack_simple_client - from package: jackd1
/usr/bin/jack_simple_client - from package: jackd2

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

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