dahdi_speed: 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/sbin/dahdi_speed - from package: dahdi-tools

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

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