This is done in mission 5 "The Royal Conservatory". You must do this before changing out the lenses/fighting the target. Head up to the second floor of the main building (with all the witches), after passing the walls of light. You should find a monument with blue markings all around the ground. Use the lever in this area to activate the voices. You will need to do this three times, the first time will alert any enemies in the area, so prepare to fight.
Here is a video guide with English commentary!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUK8mXBAnaqv...