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You can find the next key on the same floor as the assembly hall, the main character tells you this when you leave the assembly hall. All keys and diaries make the same sound. Therefore, you can navigate by the sound you heard from the first key and the first fragment. To hear the sound, you need to get quite close, this is done so as not to destroy the overall atmosphere. Therefore, you should carefully look into each room, focusing on your previous experience with the first key and the first fragment of the diary. Also, the main character sometimes voices clues in his own voice.