I did the CASA hearing test a few months ago and it is basically a case of shutting you up in the Dr. Who telephone box, sticking the ol' headphones on and playing various frequencies at you. Then the girl doing the test screws up her face when she gets blasted by a wall of garlic when she opens the door as i had over done it slightly at dinner time!!!!
I know you dont need perfect hearing and there is plenty of scope. There is even a part on the medical form for the Dr to fill out if you fail the hearing test. They then give you a 'whisper' test, where they stand at the opposite end of the room and whisper words to you and if you can hear them you're fine. It's basically only for the IFR portion of flights to make sure you can hear beacon idents etc.
Usually during the test they aren't too bothered and you can see them pressing the button. Only problem there is that there will be frequencies you shouldn't be able to hear so....................
Hope this is some help.