Who can explain how an electrical signal tells a speakers what sound to produce or how many hz to vibrate ?

The most important part of your question is that an electrical signal consists of current or electron flow. There is a perpendicular magnetic field to current that will affect the magnet of a speak and make it move as the current varies. The electrical signal for a speaker is a sine wave with a frequency, the range of frequencies that are either felt or heard range from 20 th 20,000 cycles per second. The speaker magnet will actually shake at that rate to shake the speaker diaphragm for acoustic air waves.