In RML, it was a well-known fact that when it came to resisting crowd control or negative effects, boss monsters were without equal. As a hardcore raid boss, and based on the knowledge gained from the fight so far, Vanaheim concluded that Ooudamu's resistance level was somewhere between 70-80%.
With such a massive resistance percentage to crowd control, skills that should normally last 3 seconds, would only stay in effect for 0.6-0.9 seconds. As for the skills with a lower duration, they were practically negligible!
Therefore, when Izroth stated that the voice magic would remain disabled indefinitely, how could Vanaheim maintain a straight face? But, once she took into consideration that Izroth had yet to deceive her, Vanaheim reluctantly accepted his word at face value.