The volume button on the front of the computer (labeled with a speaker icon) is stuck in the down position, and when the volume reaches zero, the menu bar flashes. To verify, open up the Sound control panel, and turn the volume to maximum. You will see the volume go all the way back to zero, with a beep at each increment, and the menu bar will start flashing again.
The solution is to press the sound button a few times to see if the button can be freed. If it cannot be changed, then the system will need to be repaired.