Hypnotherapy isn't usually carried out via tapes. Such tapes are only used by a few genuine practitioners. They're more commonly sold by companies which claim that their tapes that produce the same results as by consulting a professional hypnotherapist.
With regard to the overall effectiveness of such tapes, 'the jury is out'. (i.e. there are some people who say that they're totally ineffective, whereas there are others who claim that the tapes have changed their lives).
However there has been scientific research into the theory of 'sleep learning', which has analysed the brain activity of people who've been subjected to audio recordings while asleep. The research has conclusively proved that the brain does not recognise the auditory signals from such recordings.
So it's debatable as to whether hypnotherapy tapes work while the subject is awake, but it's proven that they can't work while the subject is asleep.
A while back i bought a CD that was meant to help you lose weight - I put it on for several nights in a row, which is a good time not to be interrupted, or distracted, and I never got further than listening to the first quarter of an hour, as I was always asleep after that!! - and no, I didn't lose any weight either!