What secrets does the organ in the Gethsemane-Kirche hold?

Mission Berlin 12 – Church Music

Anna has 65 minutes left. She figures that the music box is the missing piece of the church organ. The woman in red appears again and demands a key. What key?

Anna hears a familiar song. The church organ is playing the same melody as the music box. As Anna approaches the organ, she overhears the sexton explaining to a visitor that the organ has been fully restored but has been missing a part, ... ein Element ... since the construction of the wall. Anna realises the box is the missing link and adds it to the organ. As the organ starts playing a trap door opens and Anna finds herself standing in front of the woman in red. She loses another life and only has 60 minutes left.