Michel Gondry on Le voyage en Ballon:

“The director, Albert Lamorisse, was a helicopter operator, and he had
invented stuff to shoot from. In this one he had his son in a basket hanging under the real helicopter. It’s not like studio stuff. It’s really flying. It’s under the helicopter; it’s always super windy and they have to change the sound because when you travel in a balloon, you don’t have wind because you move with the wind. So it’s very quiet. They had to post-direct all the sound, but it’s dubbed very roughly—that gives it a very dreamy quality.”