The film is remarkable for its creative use of sound in such an early period - the first all-talking movie had come out only shortly before this, and most other directors were concerned simply with providing audible dialogue and little else.
Mamoulian not only used complex background sound effect but also used them creatively and non-realistically in the case of Kitty's delirium. The technical aspect was very advanced for the time. The scene in which Kitty sings while her daughter prays was apparently the first time anyone had ever used two microphone at the same time. (This is generally noted about this scene, but in fact there would be no need for two mics. A much more likely candidate is an earlier scene in which Kitty is sitting on the floor surrounded by photos and papers and is singing: there is then a diagonal 'wipe' to a dialogue scene in another set, while the singing continues. This was probably filmed simultaneously with two cameras and would have needed two microphones.)
He also made his staff move the large box in which the cameraman was enclosed during shots to provide tracking with sync sound - unheard of at the time.
Most of the sound effects were created in the studio at the time filming of the action took place. The train moving off is plainly an artificial sound effect, and most of the traffic sound is horns and motors in the studio. Despite claims elsewhere that the scene in the railway station contains sync sound it doesn't - indeed the filming of that sequence was visibly done with a hand-cranked silent camera, the sound being created afterwards. The scene near the end in the subway station is indeed local sync sound, done quite extraordinary well considering the equipment available at the time.
The music was all done live. The extended scene between April and the sailor in the café is all one extended shot because the band seen at the opening of the shot was actually playing in the studio at the same time - indeed the music almost swamps the dialogue. There is sophisticated use of the stage music early on, keeping it in the far background during dialogue in the dressing room - again, advanced use of sound for 1929.
