Hypnosis is similar to subliminal messaging in that it too aims to slip past the conscious mind and engage the unconscious.
Hypnosis works by encouraging deep relaxation and a trance-like state. In this state, the brain produces fewer Beta waves, and more Alpha (and sometimes Theta waves). These waves are associated with a more active subconscious mind. This drowsy trance like state is a normal state the brain slips into many times a day. There is nothing strange or fantastical about trance. In this trance state, the unconscious mind is able to accept suggestions more readily.
Before continuing it is important to point out that so called "stage hypnotism" while more widely known, has very little in common with hypnosis.
Constrary to popular belief hypnotherapy requires active concentration on the patient's part and engagement with the guided imagery and oftentimes communication with the therapist while in trance to be effective. Post hypnotic suggestion is a small facet of hypnotherapy, and is widely considered to be one of the less effective modes of hypnotherapy.
So it is important to distinguish between hypnosis mp3 recordings and hypnotherapy as conducted by a properly trained hypnotherapist. Of course, hypnosis mp3s do not deliver this experience so it is not reasonable to equate the potential effectiveness of a hypnosis mp3s with a individual attentions of a trained hypnotherapist. Rather hypnosis mp3s facilitate guided meditation, metaphor and post hypnotic suggestions. Hypnotherapists often provide such recordings to supplement therapy as they are excellent means of teaching self-hypnosis.
As one would expect hypnosis mp3s demand a huge time commitment. In the absense of a hypnotherapist, the subconscious requires a lot of focus and repetition before it integrates the new learning. It can become very boring to regularly follow the same recording. Listeners can become restless and it is easy to see why this would negate any beneficial influence.
Simiarly, subliminal messaging is not as effectivess as a short series of consultations with a hypnotherapist.
However subliminal messaging has a very clear advantage over hypnosis mp3 recordings. It does not require the brain to be in this state to work.
It only requires:
the conscious mind must be unaware of the message
the unconscious mind be made aware the message exists
the message is repeated enough times to prime behavioural changes
The lister is however unaware of the repetition required.