This is an optional mission objective that you get at Plaza of Zeal

To locate the Vox Cipher, find this building:

Plazaofzealvoxcode1.jpg


that says "Closed by Order of Jeremiah Fink"

and request that Elizabeth open the gate (requires 1 lock pick)

Find the secret message at the rear wall of the clockmaker shop:

Plazazealvoxcode2.jpg


Go to the Good Time Club:

Thegoodtimeclubbioshockinfinite.jpg


and head to the basement.

There is a blackboard that has Chen Lin's name written on it at cell #9:

Chenlinblackboard.jpg


Have Elizabeth pick a lock on this door:

Elizabethopendoorchenlin.jpg


Enter a room that has a "disposal" sign in it:

Voxcode2screen3.jpg


Code Book Location: Find the Code Book on some posters in that room:

Voxcode2screen4.jpg


Go back to where the code is, Elizabeth will decode the message, use the cipher to crack the vox code.

The vox secret code reads "The Broken Clock Tolls at Midnight". To the right of the code there is a clock, set clock time to midnight (both dials to "12" as seen below)

Clockmidnightbioshockinfinite.jpg


This will reveal a secret safe behind it with an Infusion and some other stuff.