I agree with Bob, I think you should ask them questions just to check whether they are listening to their playing. I even ask them to give a score (1-10) or how good (or bad) it was, and its surprising how open they are about their playing and mine! So I ask them, what do you think you should do about this so and so passage? I also praise them when they seem to listening to their playing. Like, if they start off a soft passage with a loud bang, and suddenly say oops sorry, can I start it again, it was too loud! I just say GOOD, you are listening to your playing! I think this should head them in the right direction, that music is all about ears first.