Hi Stevie:
From what I've been gathering from different discussions on the matter of "position while listening", you could probably stand on your head and benefit from the entrainment - by which I mean to say that as long as you are comfortable (and if you fell asleep you must have been), your body is relaxed and receptive, and therefore, as long as the earphones stay on your ears, you will be getting the entrainment effect.
I think the idea behind the "sitting position" relates to the flow of energy most dyed in the wool meditators consider optimal. From my own experience, most of the time I do holosync in a semi-lying down position in my recliner, and I definitely feel a good energy flow, as well as have experienced great results along the lines Bill Harris has expounded on in his writings. So I don't particularly consider there is any particular right way for position of body while using the Holosync program.
So, if you are comfortable while you meditate, regardless of position, it is right for you.
Don't be put off by falling asleep either. It could be since you used it in the evening you were tired enough to sleep. It could also mean that the entrainment is really working for you (regardless of any physical sensations), because if there is lots to clear up in the subconscious mind (and for most of us there is, or we wouldn't be seeking ways of making ourselves better), then "sleeping" is sometimes the best way for it to get the clearing process underway.
Did you notice if you had any dreams? I have found starting a new level, and even starting the lower level CD's within a level, I have increased dream activity for about a week - sometimes for the whole level. Dreams are often part of the process of clearing out "garbage" that's stuck in there.
My advice would be not to push yourself too hard on "listening" - do what is comfortable and gradually work your way into it again. Don't worry about your position or sleeping. Be comfortable. When you are ready to move forward to the next level, you will probably find you are staying awake, or waking more often during the sessions. I would also suggest keeping a diary, starting now, of where you are at in your life, and noting down anything that seems to come up for you, either from your daily thinking, or from your dreams, as this will be your guide as to whether or not Holosync is working for you. Many people don't experience physical sensations - and they still experience major shifts in their lives over time while using Holosync.
Cheers!
Unis