how does auron....

[Little Blue]

kinda on topic but not really
when they fall through the ice where did all that crap come from? had it always been resting on sins back or had it been dredged up from the lake floor

It was probably ruins left from the Machina War, like the ones at the bottom of the Moonflow.

If that was a frozen lake, then the real question is, when they broke through, how could they breathe and talk?

All that crap, as you put it, was ruins left from the Machina War.

As for why they can breath, the water is down by their ankles if you'd care to view the scene again... Though then the FMV of Sin sleeping does give an odd sense of perspective... *shrugs*