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

John Olinger can!

I think fluctuating currents in wires generate fluctuating magnetic fields nearby which attract and repel magnets affixed to vibrating membranes, which push air and make sound.

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.

Not me. I could t explain 99 percent of technology to aliens from another planet, I would just shrug my shoulders and say I dunno.
I can show them how to hook up speaker then they are on their own.

The amplifier does that. It switches the signal on the power wire between positive and negative using a variable frequency.

Sine waves and Fourier transforms (most stuff is mp3 these days) is as BoringAF and Im flat out.

Current, voltage, varying sinusoidal audio signals, roughly 20 to 20000 hertz

Black magic!