Since he cannot advance further, Booker DeWitt must find the security gates switch inside the Wardens Office, but for that he needs to go through The Atrium.
Even if The Atrium isnt stretching on both floors of the area, the collectables guide considers that the BioShock Infinite collectibles found along the way belong to the current chapter, until you use the switch from the Wardens Office.
When you step outside the elevator you encounter another Boy of Silence.
Sneak by or kill his minions, but when you can, head left (your objective is in the opposite direction), because around the corner you can find the first BioShock Infinite The Atrium Kinetoscope next to the second column/pillar.
Return to your objective to reach the area with the observation room, past the Boy of Silence (you can see a Tear inside).
The waypoint shows that the right path is on your right, but for now head left, inside the morgue/crematorium. The first BioShock Infinite Voxophone in The Atrium is in a coffin next to the furnace.
THE VALUE OF CHOICE; Elizabeth
After you deal with the third Boy of Silence you end up in the area thats essentially behind the elevator you used to reach The Atrium.
There are two guns at the top of the stairs and some enemies around, but when the path is clear you follow the waypoint inside a corridor. At the end of the corridor is the second BioShock Infinite The Atrium Kinetoscope (just before the room filled with heads).
In the next room you should find the fourth Boy of Silence. You need to get behind the column/wall thats next to him, preferably undetected.
The second BioShock Infinite Voxophone in The Atrium is on the table thats behind the column.
Continue forward until you find a room with a movie projector.
You have to go through the broken observation window, but before going around the left corner, take a quick look on your right to spot the third BioShock Infinite The Atrium Kinetoscope.
After you discover the previous collectible, you have to go upstairs. Before doing that, look closely near the staircase for a movie projector (without a film).
The third BioShock Infinite Voxophone can be found right next to the movie projector.
A LAST CHANCE; Elizabeth
Head upstairs, inside the Wardens Office. On the chair inside the office is the fourth BioShock Voxophone, and it is a good idea to collect it before you hit the switch.
SMOTHERED IN THE CRIB; Elizabeth
When you use the switch to open the security gate you trigger a battle and the next chapter named Warden’s Office.