If you're listening to hypno audio files, and you relax to a point where your jaw lets go and the mouth opens, you're doing better than most who will consciously keep that tension tight and then wonder why they're not having a good experience.
Protip: if the jaw relaxes, if the mouth opens, note it and then let it go because it's a good sign you're doing well.
If you need to swallow at any point do it, it's not going to break the trance or something. It IS a shame that more 'tists don't tell the subjects in the induction or the file itself at some point that it's ok to move, to make yourself comfortable even after the trance begins, swallow if needed, etc.
Most 'tists don't do these things, and I don't understand why some people just don't seem to give a fuck but I suppose we're all different.
Breathe however it's most comfortable for you at any given time, if you're putting your conscious awareness on it that much it's ok because that's just another distraction for the critical factor aka the conscious to deal with while the suggestions get through easier to the unconscious.
tl;dr Stop fucking worrying about it and relax, you're doing fine.