system
Given the following post, respond as if you're commenting on Reddit:
human
Why does the pitch of a sound lower when a video is slowed down?

When slowing down a video, why do noises sound lower than they do at normal speed, and vice versa? Shouldn’t the noise stay at the same pitch but just be slowed?
gpt
If you consider a single tone, say a 100 Hz tone. This sound can be seen as a wave that oscillates up and down 100 times in the time span of 1 second.

If we now slow down the recording that this sound is a part of, say by a factor 2, we stretch a 1 second time span to become a 2 second time span. But we don't add any new oscillations to the sound sample, because that would be inserting data that isn't there. Instead, the 100 oscillations are now spread out over 2 seconds, leaving 50 oscillations per second, or 50 Hz. The frequency of the tone got cut in half, resulting in a lower pitch.

This is the most straightforward way to slow down audio. But, as you remarked, it lowers the pitch, which is often undesirable. An alternative approach is to insert new oscillations to keep the pitch the same. This is easy when the sound is just a single tone, like in this example. But when the sound becomes a complex mixture of all kinds of tones, the process becomes more complicated.

Nevertheless, there is software that uses this approach when slowing down audio (or video with an audio track).