what was unusual about the sound production of citizen kane
Welles used techniques from radio like overlapping dialogue.
Citizen Kane
Stewart said that Hollywood films never deviated from a basic pattern of how sound could be recorded or used, but with Welles "deviation from the pattern was possible because he demanded it." Although the film is known for its complex soundtrack, much of the audio is heard as it was recorded by Fesler and without manipulation.

Welles used techniques from radio like overlapping dialogue. 
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Citizen Kane [SEP] Sound
He also used different "sound perspectives" to create the illusion of distances, such as in scenes at Xanadu where characters speak to each other at far distances. Welles experimented with sound in post-production, creating audio montages, and chose to create all of the sound effects for the film instead of using RKO's library of sound effects. Welles used an aural technique from radio called the "lightning-mix". Welles used this technique to link complex montage sequences via a series of related sounds or phrases. For example, Kane grows from a child into a young man in just two shots. As Thatcher hands eight-year-old Kane a sled and wishes him a Merry Christmas, the sequence suddenly jumps to a shot of Thatcher fifteen years later, completing the sentence he began in both the previous shot and the chronological past.