philips car 400 not hearing beep sound when pressing AS key?

I have a problem since i can't hear a confirmation beep. I'm switching on the set and keeping AS key pressed but I don’t hear a confirmation beep in order to set the code ???? Please can you help on this matter?
Radio type: CAR 400
Serial no: GM0400T1388143
Code: 9654
It used to be 9331 but it has been changed and now it works with 9645 since I unlocked it many time. The problem is that I can’t hear sound BEEP when i press and keep AS key??? Any solution?