Via Sven E Carlsson I find this fascinating description of the process of sound editing in the last film of Michael Moore: Fahrenheit 9/11
"Another key to the success of the sound edit was the tireless efforts of Carl Deal, the chief archivist. His job was to make sure there was plenty of archival footage for Moore to cut into the movie. “Along with the archival footage comes archival sounds,” Rizzo says. “If a piece of archival news footage had really bad sound, we looked to Carl to find us something better, another source for the same piece of audio.” Rizzo recalls that even though Deal was often able to come through, there sometimes wasn't a better sound source to be found, so other elaborate audio restoration actions had to be taken.
One of the Fahrenheit sequences that was particularly important to Rizzo, and particularly moving in the film, was the 9/11 montage. Cinematically, it was decided that since Americans had seen the Trade Center footage hundreds of times, the film would allow the sounds to carry forward the images.
“We wanted to start off with a couple of seconds of quiet, of city ambience,” Rizzo explains. “The last sound before our world changed. Then you have all this confusion. This mass panic. Some people are screaming, some people are crying. A little bit of radio clip. Every sound that you hear [in the movie] came from recordings of 9/11. No library sounds were used. The challenge was to make it as powerful as possible without doctoring it up. Make it as adrenaline-flowing as it really was without hypersensationalizing it. Make it real.”