On this page of our guide to BioShock Infinite you will find the fourth part of a detailed walkthrough of Chapter 35 - Operating Theatre. Booker and Elizabeth will begin their escape towards Comstock's airship. The heroes have to reach the elevator, and on their way you may find a voxophone and an infusion.
Go back to the first floor of the operating block and ask Elizabeth to break the lock on a door located between the corridors leading to turned off generators (the lock requires 3 lockpicks).
Explore a newly unlocked room and check the desk to find some silver, Voxophone #76 and Infusion #24 (last one in the game).
Return to the hall with the closed gate, wait for Elizabeth to open it (she won't need any lockpicks) and enter an elevator located on the other side of the gate.