I believe that the explicit message is that the author’s inspiration for Merlin’s voice was the owl outside his window.
In the excerpt from the article "Vision, Voice and the Power of Creation: An Author Speaks Out," T. A. Barron states that he had difficult time finding the right voice, but at the end, the inspiration was the sound of an owl outside his window: “I finally heard the voice of Merlin thanks to a surprising source: the haunting, mysterious hooting of a great horned owl outside the window of my Colorado home.”