Is it possibe to be sound asleep and pass gas? I think that the process of passing gas takes muscles and voluntary control in order to complete the process which I would think takes a conscious mind and not a sleeping one.
You darn tootin’. I’ve woken myself up with some particularly large sonic purges, not to mention my wife. It takes muscle control to keep them in, like pinching the neck of a balloon. Think barometric equalization. You relax too much and the sneaky things will let fly if you give them half a chance.
My guess would be that they occur during RAM sleep, or Rapid Anus Movement.
Jesus, my dad does it all the time.
I’ve noticed that the difference, between sleeping and waking flatulence, is that it just escapes sleeping people “pffffft…” where as waking people forcibly eject it “braaaaap.”
In other words, sleepers poot, wakers fart.
Have you ever seen/smelled a sleeping dog?
Why were you sleeping with your dad, you sick bastard? 
Do you always need to forcefully expel your farts? As was said before, it takes effort to hold them in. Letting them out merely requires relaxing the sphincter. Relaxing muscles is pretty easy to do when you’re sleeping.