When watching videos/listening to audio that is sped up the voices are always higher than normal. Why is that?
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To get the same number of cycles in a shorter time you have to have more cycles per second = the definition of higher frequency.
Sound is vibrations in air (or other materials). To our brains, more rapid vibrations are interpreted as a higher pitch. The lowest pitch we can hear corresponds to about 20 vibration cycles per second, while the highest pitch corresponds to about 20,000 cycles per second.
If you speed up a sound recording, you speed up all of the vibrations, making them higher pitched.
Sound of any specific pitch can be literally represented as a sine wave of air movement over time ([here’s a math program](https://www.wolframalpha.com/input?i=play+sin%28pi*220*t*2%5E%280%2F12%2B1%29%29+) using that fact to play an *a* note). Changing the properties of the sine wave changes the properties of the sound. Making more of the sine wave happen in less time (speeding it up) makes it look identical to a higher pitch’s sine wave ([here’s the same math program](https://www.wolframalpha.com/input?i=play+sin%28pi*440*t*2%5E%280%2F12%2B1%29%29+) playing a note an octave higher. Notice that the ‘time’ on the graph is half as large).
Sounds with non-specific pitches are the same, just instead of a sine wave it’s a lot more complex of a line graph. Speeding it up still shifts the pitch.
The faster it is, the higher frequency it will be, and that is higher pitch.
If you threw a mouse really fast, it would be a very high pitch squeak. If you tried that with an elephant, it would be very slow and deeper rumbling bassy sound.
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