A new Boy of Silence will suddenly appear behind you after you open the gate on the ground floor (ready for a heart attack?) and in this case he will summon a group of armed guards instead of the usual puppets with masks.
Fight your way to the atrium where you'll encounter another group of guards. Enter an elevator after the battle, pick up Voxophone #75 and travel to the ground level.
Walk through a recently opened armored door and head further info the building. Listen to a new conversation with the Lutece siblings met along the way and then enter the snow-covered hall. You'll finally encounter Elizabeth near the door at the end of the room. Talk to her and give her your hand to advance to the next chapter.