Those beeps you hear in recordings of astronauts in space have a name — Quindar Tones:
First, let’s be specific about those beeps: there’s actually two different beeps that happen, one a sine wave tone at a frequency of 2.525 KHz that lasts for 250 milliseconds, and one that’s a sine wave tone at 2.475 KHz, for the same duration.
That first and slightly higher tone is called the intro tone and the lower one is the outro. As their names suggest, one is for the start of something, and one for the end.
What that something is related to how the CapCom — that means “capsule communicator” which was what they called the ground control team member (usually an astronaut) who was in charge of talking directly to the astronauts on the spacecraft. Having one person designated to communicate with the astronauts helps reduce any possible confusion and cross-talk.
Since the CapCom would be in the busy, noisy Mission Control room, they’d want to choose when to open their microphones to talk to the spacecraft, so NASA used a push-to-talk (PTT) system.
It’s like how a CB works, if you’re as miserably old as I am and remember that — you hold down a button while you talk, and let up when you’re done.
This is normally not a big deal to implement, but the space program had very unique requirements. In the setup that NASA developed, which used tracking stations all over the world to keep in near-constant communication with the spacecraft, the audio from CapCom to be sent into space was transmitted to the various stations across the globe via dedicated telephone lines.
These lines were just for voice audio — if NASA wanted to send control signals like transmit on and off, they’d need to run a whole parallel set of wires, which would be expensive. So, they came up with a solution: use the same lines for control signals as well!
Because the lines were optimized for human voice audio, the control tones had to be within the range of normal human speech, which is why the tones are audible.