If the path to the hypnotic was through boredom, I guess a deep monotone voice would be preferred. I think that Ernest Hilgard is responsible for this myth because he was a prominent researcher applying scientific principles to the field of hypnotism. Unfortunately for Ernest Hilgard, he chose to hypnotize people with an induction tape, with a person reading out an induction in an intense monotone voice, and had only success with 10% of the subjects. Since hypnosis is natural, anyone with a normal functioning brain can be hypnotized. Hilgard’s studies then showed that such a method for inducing trance wasn’t very effective.
Another thing to consider is the skills of the hypnotist. Even if a deep monotone voice was better for inducing the state of trance, there remains the question of how the hypnotist handles the problem when the magic is present. A classical hypnotist would rely on direct suggestion, whereas a modern hypnotist (sometimes called a hypnotherapist) would apply techniques that have proven more helpful in overcoming various problems.
When you consider a hypnotist’s voice, you should think if the person sounds like someone you would like to help you. If the hypnotist has a voice that sounds very weird to you, it’s better to find someone else than consult this hypnotist and not take him seriously. A deep monotone voice is no guarantee for you; you should take other measures to find a qualified hypnotist or hypnotherapist.