This is an optional mission objective that you get at Plaza of Zeal
To locate the Vox Cipher, find this building:
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:
Go to the Good Time Club:
and head to the basement.
There is a blackboard that has Chen Lin's name written on it at cell #9:
Have Elizabeth pick a lock on this door:
Enter a room that has a "disposal" sign in it:
Code Book Location: Find the Code Book on some posters in that room:
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)
This will reveal a secret safe behind it with an Infusion and some other stuff.
