The current from the amplifier is fed to the voice coil which is wound round a paper former fixed to the speaker cone. This former fits over the core of a tubular magnet. The current in the coil makes it a magnet and so it is affected by the field of the speaker magnet. The force on the coil makes it move at right angles to the magnetic field – that is in and out of the paper in the first diagram. Since the signal is a.c. the force on the coil is changing and so it moves backwards and forwards, this in turn makes the speaker cone vibrate giving you the sound.