I have a macbook pro and suddenly my internal speaker not working do you know something about it ?

Try an SMC reset:

Resetting the SMC (System Management Controller) on Intel-based Macs:

http://support.apple.com/kb/ht3964 and

http://support.apple.com/kb/HT1237?viewlocale=en_US

Resetting your Mac's PRAM and NVRAM:

http://support.apple.com/kb/ht1379

If the afore mentioned by Klaus 1 does not solve the problem, look in the audio output port for a red light. If there is one visible, insert a phone jack a few times. If that fails, insert a tooth pick and jiggle it. If that fails have a repair facility do it.

There is a switch that you are trying to trip that will then send the audio to the internal speakers. A red light indicates that only ear phones will get the audio signal.

Ciao.