I was experimenting with the subtitles today too - I had never heard the "c'mon lie back" before and was totally shocked to see it in the subtitles, but I turned the surround on as loud as I could and it IS there, but faintly. I am also in the "I'm sorry, s'alright" camp. To me, it's two I'm sorrys followed by an It's alright, and all coming from Jack. I know it makes more sense if Ennis says I'm sorry, but his lips don't move at all there.
My interpretation is that Jack is apologizing for maybe pushing Ennis into something that he wasn't totally ready for, but then saying "It's alright" as a way to calm the "skittish horse" as Paul called Ennis. Followed by "C'mon, lie back". Obviously this is something that everyone will interpret for themselves (proven time and time again on every board we've argued this point on, lol). That's just my take on the scene.