There Was A Recording Issue
The voice-over by Morgan Freeman was actually recorded before to filming and was played out loud during production to set the rhythm for each scene. In Iowa, Freeman recorded the full narration in 40 minutes. However, something went awry, and the track had a significant hiss that the sound professionals in Los Angeles were unable to remove. As a result, they had to re-record the entire thing in a proper studio, which took three weeks to accomplish.

There Was A Recording Issue