The Voice of Elex is one of the main quests that are unlocked at the beginning of the fourth chapter in Elex. This quest is part of the preparations for the attack on the Ice Palace and during it you must assist Thorald in an experiment with Elex.
How to unlock this quest: Speak with Thorald in Abandoned Cliffs.
During your meeting with Thorald you can ask him about the progress of the experiment with Elex. You learn that Thorald needs a help from a powerful mage. Tell him that Caja can participate in the experiment. Meet her in the team camp and ask her for help (1400XP). Return to Thorald and allow him to start. Zardom's sudden appearance makes the experiment end in failure (3500XP).
Speak with Thorald (7000XP) and go to Company Premises: North Abessa. There, you must find unconscious Caja who will join your team again. A short conversation with the woman ends this quest (1400XP).