Ever wondered where that knocking sound comes from when you get an extra ball or another game (for example, due to matching the last two numbers on your score)?  It comes from a rather simple mechanism consisting of a coil with a plunger that basically knocks in to a piece of wood or metal mounted to the side of the box, or it just bangs in to the side of the box directly.

(click picture to enlarge)

Here's a typical knocker used for an Extra Ball:

How about those chimes that you hear when your ball is scoring points?  Same kind of thing.  A coil has a plunger that taps a metal plate that acts as a "gong" or "bell" and in some cases a literal bell is struck (see below).  Several coils can be used to indicate different score values by hitting different sizes of metal bars and making different tones.  Take a look at this old EM's bell with the Extra Ball knocker mounted right next to it.