Whatever words are being said (I'm in the three "s'alright"s camp myself), I'm as sure as I can be that Jack is the only one talking, with Ennis remaining silent. You don't see Ennis' mouth move, and each time you hear speaking it sounds like exactly the same voice, pitch and everything. And we clearly see that it is Jack speaking by the third word, so this would lead to the conclusion that it is Jack speaking each time. Just my opinion, of course.