After getting past the first Wall Of Light in Mission 5: The Royal Conservatory, you will reach an area with a watchtower and another Wall Of Light. Proceed through that area to reach a large building with a lot of Witches (same building where you have to defeat the main target). In the main hall, go up the first flight of stairs to find a shrine on the balcony with blue glowing markings on the ground. It also has drawings of blue aliens and a machine with a switch. Activate the switch to trigger the voices. Activate it three times to listen to all the voices and get the "Oracular Echoes" achievement. Make sure to wait there until they are done talking.