It turns out, more often than not, radio waves were recorded from each planet, and scientists (or physicists) transcribed those waves into sound waves. Any other time this data isn’t taken from radio waves (or electro-magnetic activity), it was taken from either plasma waves or, in the case of a black hole, gravity waves.
I had never heard of gravity waves before I found the “audio” clip from a black hole. Apparently, the sound waves were derived from the “gravity wave” pattern coming from the black hole. In my research I found that gravity waves were very recently discovered (or proven to exist, if you will, because the experiments/hypotheses have existed for a while).
In reverence to the movie, I made my process book in the shape of the infamous Obelisk from the movie, which I thought to be a great metaphor for the idea of how little we may know about our universe. I also took some inspiration from the "trip" scene later in the movie.