What is it inside a computer that gets the electrons in the wire leading to a speaker wiggling back and forth?
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A power transistor in the speaker driver circuit.
There is a source of current, the power supply, and the speaker driver takes some digital waveform from the game or movie and converts that into an analog signal, which power transistors amplify until there is enough current to drive the speaker coil.
I believe it comes down to electromagnetic fields that impart their effect on wires.
Here’s a moderately long video that helps explain what is happening. It’s different than most people are taught.
an electromagnet. The signal turns an electromagnet on and off, which then pulls a piece of metal towards it then releases it, that piece of metal is attached to the vibrating part of the speaker, making it vibrate as it gets pulled and released.
Inside the computer the part that turns data into an electrical signal that looks like a sound wave is called a digital to analog converter(DAC), there are a few ways these can work, but two common techniques are:
having many pins driven on or off, which are then weighted differently and added together. (R2R)
having a single pin turn on and off rapidly with the proportion of on to off determining voltage level, then having the signal smoothed out. (PWM, Delta Sigma)
Once you have this analog signal, it is amplified to be strong enough to drive a speaker. Class D amplifiers are common these days, but you’ll likely find class AB in older or higher end equipment. Class D works like the second kind of DAC I mentioned, it’s more efficient because the transistors are either all the way on or all the way off. Class AB has transistors that are partially on providing the amplification, which wastes more output power as heat.
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