Hi there, as is obvious by my nickname I actually teach HypnoBirthing, although I've also used it myself three times (before training too) - I've tried all of the methods over the years, from CDs, to sessions with hypnotherapists (not actual HypnoBirthing practitioners) to trademarked HypnoBirthing and have to say that the actual HypnoBirthing is very different and worked so much better for me (I had epidurals in my births with just the CDs and the hypnosis).
Most people dont understand the differences, in HypnoBirthing (the Mongan Method) classes you actually cover lots of regular antenatal stuff (so you dont need to go to any other classes if you don't want to), you cover forming a birth plan, watch loads of DVDs, are taught about the history of birth and why people think it's painful (we call this "de-hypnotising" you), massage, breathing techniques, positions for birth, place of birth etc..but the best thing is your partner (usually the dad) is there too which makes a HUGE difference in my personal and professional experience.
When I just had hypnothereapy and listened to the CDs on my own at home I don't think it worked so well (well clearly it didn't as it bloody hurt and I had an epidural! with my HypnoBirths I didn't need any pain relief) as my husband wasn't able to help me in the same way as with HypnoBirthing where he was really involved.
Also the added benefit with classes is that you get the hypnotherapy in class (so does your birth partner which is great for sceptical and nervous dads!) which can be tailored to you, which obviously you don't get with a "one size fits all" CD.
Re. the voices being irritating it actually doesn't matter as in hypnosis your subconscious is non-critical, in other words it doesn't matter if it's a man or woman reading, if they are American, English, French, Spanish etc....all hypnosis is self hypnosis too and once you learn to relax correctly (another plus for the classes as your teacher can check you are doing it properly) the voice really will be irrelevant.
HypnoBirthing was started by a lady in America but actually all of the theory is English (and 100years old!) there is also a UK specific book version and hundreds of UK practitioners on here: www.hypnobirthing.co.uk
Lastly re. the expense do shop around, many HypnoBirthing practitioners have free places or reduced rates for those with financial difficulties and many offer payment via installments, my own eg are £200 paid in 4 installments of £50.
The one other thing re. expense is I always say to people "how much did your pram cost?" "how much are you spending on the cot?" etc...I don't understand why people are willing to spend many hundreds of pounds on kitting out their nurseries and buying designer prams when the one thing that really matters to the health and wellbeing of the mum and baby is the baby's birth - isn't that the one thing REALLY worth investing in? It costs less than a £1 per day every day of your pregnancy to do a HypnoBirthing course!!
Anyway, good luck, I hope you have a great birth :-) It's such a special time, I'm envious of you doing it again!