Best advice I can give it choose the LOUDEST piece of music possible :) the banging of the circuitry (nothing actually moves bar the bed) is incredibly loud.
Do ask the radiographers to constantly talk to you. Some places do, others just check you can hear them then get on with it. You need to tell them you need to have constant talking to. It helps a lot.
Do think of it as hiding under a duvet. A very noisy one. Close your eyes the instant you lie down, don't move (very very important else they have to repeat a measurement!) then you don't see anything after that.
They will give you a hand buzzer with which you can make a b. awful hooter go off outside, make the radiographers jump and get them to talk to you &/or come and get you out!
It takes just a few seconds to get you out after pressing the button and yelling. Most of the time they'll say are you ok? and if you say NO! then they'll get you out. During the scan they can't hear you talk because otherwise the banging would distract them. They can only hear you if they press the intercom button, which you can request via the hand buzzer.
If you are that bad, see if one of them will stay inside the room with you. If they hesitate, tell them the PAD has been delayed for another two years. (They ought to know what that refers to!)
If you think you have anything wrong with you, please try and have the scans as it could save you a lot of pain to have the right diagnosis.
Diazepam will help lots. So will making sure you're comfortable with pillows and foam pads under your back etc.
(I'm another person who has slept blissfully in a scanner - my record was 2h 20 mins during a quiet research scan for a student.)