The instructions for the ohm mwa sounds are different for different meditations. Irregardless of which meditations, it's valuable to remember that the sounds are not essential to the practice. The are a good addition.
During the Small Universe the Master Lin gives the sounds during the inhale, you move (he says push) the engery during the exhale. On alternate inhales you will ohm then mwa. So two centers in sequence have different sounds. Also as you repeat the cycle you use the same sound each time you visit the an energy center (eg. lower dan tien always gets a ohm).
Now you were really asking how you make the sounds. Well you can do it as Master Lin says on the inhale. For myself it doesn't sound exactly the same. Also I find I have to open my mouth for part of the sound.
So I prefer to hear the sound with my mind, as if Master Lin or I were creating the sound in the full richness. This is what I do most of the time. Definitely when doing the Small Universe. Again don't fret about the sound, just do your laps as gallen called them. I find doing them without the audio is nicer. I find the instruction and have to synchronize with the external distracting and limiting.
You can even learn to remember the feel of the sound. I like the Sword Fingers and Thunder Palms meditations for this. Maybe because I like working with the middle tien (Heart). However, I can feel the vibrations very powerfully, in the heart, when doing the sounds. I can even feel them when just thinking the sound. Not as strongly, but I like to keep my tongue well connected with the roof of my mouth.
My encouragement is to do as many of the meditations without the audio assist as you can. I've got all the material on my mp3 player and I've even mixed a few meditations to follow the meditations formats at the level III retreat. I really like the meditation musics pieces, especially after after doing one of the other meditations (with the closing removed). I find the parts of the audio just vibrate through my whole being. Which is when I become aware of the audio again. It's such a different experience to listening to the music when not in meditation.
I strongly recommend doing as much SFQ in silence, or with supportive non-conscience demanding audio, as you can.
You are perfection,
Iam2