I have noticed that on my PowerBook G3 Series (Bronze keyboard) sometimes the volume buttons don't make an audible beep unless I increase the volume to the maximum. Why is this?

In certain circumstances the system software may think that a CD is playing when in fact it's not. For example this can happen after a CD has played all the way to the end and stopped on its own. In this condition, the volume buttons will not make beep sounds when pressed unless the volume is increased to the maximum. Visual feedback is still provided, however.