If the path to the hypnotic was through boredom, I guess a deep monotone voice would be preferred. It is my guess 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 choose to hypnotize people with an induction tape, with a person reading out an induction in a very deep monotone voice, and had only succes with 10% of the subjects. Since hypnosis is a natural state, anyone with a normal functioning brain can be hypnotized, and what Hilgards studies then showed, was that such a method for inducing trance really 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 still remains the question of how the hypnotist handle the problem when the trance 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 useful for overcoming various problems.

When you consider the voice of a hypnotist, you should really consider if the person sounds like someone who 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 consulting this hypnotist and not being able to take him serious. A deep monotone voice is no guarantee for you, you should take other measures to find a qualified hypnotist og hypnotherapist.