The adjective that supports the author's explicit message about his inspiration for Merlin's voice is mysterious.
The author says that his inspiration came out of nowhere, during the late night (pre-dawn hours), from an owl's hoot. All of these seem very mysterious to me.
Young, new, and whole are not the adjectives to describe the author's inspiration, but rather Merlin's voice, which is not the same.
Hope this helps! Have a blessed day!