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okay so i quit physics after a year and i never heard a lecture on solid state physics, so take everything i’m about to say with a grain of salt :3

there is this physical effect called the piezoelectric effect where certain crystals like quartz for example will produce an electric charge when you squeeze or stretch the crystal. and they can easily produce a couple thousand volts that way (just not a lot of power) which is why they get used for things like lighters to produce a spark to start the flame (not the ones with the wheel that you turn to produce a whole lot of sparks, that’s just flint afaik, but the ones you press hard and then they suddenly give way and produce a flame)

the thing about the piezoelectric effect is that it also works in reverse. if you apply a voltage to such a crystal it will deform itself, ie stretch along a certain axis. and this happens really fast which is why they use it to make high frequency ultrasound. regular speakers will be too heavy and slow to produce 100s of kilohertz.
however, if you apply the voltages at lower frequencies, you can also produce audio signals that humans can hear. as you can hear in the clip, they sound terrible (and here’s where i am speculating a bit but i think it should be true) because piezo buzzers (ie devices that use piezoelectric crystals to produce audio signals) can’t really produce nice sine waves. the form of the crystal is fairly binary as in either stretched or not stretched, on or off. so the sound waves it produces look very square. in comparison, regular speakers can produce sine waves because the speaker is basically a pendulum that swings back and forth. so you don’t really want to use piezo buzzers for headphones or anything, but they have two advantages: they’re cheap and reliable. and that’s why they’re in the computer in the video: for a long time and up until today, manufacturers would often put these little buzzers onto computer motherboards as an error reporting tool. if something went wrong and say your memory doesn’t work correctly, you won’t be able to boot. the motherboard can’t communicate what the problem is to you on the screen, because it would need memory to do that. so instead they add this little buzzer that probably costs them like five cents which in those cases can make some specific sounds. and you or a repair shop can go look up the sequence of sounds in the manual to check what the problem is and fix it. :3