Here is why slow motion sound is mostly fake.
Whether you are watching the latest Hollywood blockbuster movie or diving into the depths of YouTube videos, when you stumble on some incredible mind-blowing slow motion footage, the sound you hear is mostly sound design and not necessarily the actual sound captured and slowed down.
It all boils down to what makes slow motion possible, and that is the frame rate of the camera you are filming slow motion with.
For example, most consumer-to-enthusiast film cameras nowadays are able to shoot at least 50 frames per second, which, in turn, allows you to slow it down to 25 frames per second. This half speed will slow your audio down half speed as well, as you'd probably expect it to.
The problem comes in when you start shooting with dedicated slow motion cameras that are able to shoot anywhere from 240 frames per second to 20,000 frames per second. Most cameras that are able to shoot at those high frame rates don't even capture sound at all.
And the reason is that, for sound, you also have a "frame rate" which works in a similar way to video. It's called the sample rate, however, when you slow down sound past a particular point, it sounds like a constant deep tone, and you can't make out anything that remotely resembles the visuals.
So what's the solution? Well, as anticlimactic as it may seem, it's sound design in post-production.
Take a look at the video below by YouTube channel, The Slow Mo Guys, on Why Slow Motion Sound is Fake...Mostly.