On a serious note, PUCCI (and I apologise for my slightly flippant contribution earlier), could technology help here? An overlay on the IFE could tell those that are in their seats that they would need to fasten their seat belts. For any that are asleep it's quite possible that they wouldn't hear the announcement anyway. I think that you are required to do a pass through the cabin (?) to catch anyone not strapped in, but at least those that are watching would have had warning without interrupting what they are watching too much. But then again, if they are sat down they should have their belt on anyway - I always do, regardless of how smooth the flight may appear.

And actually I've had the volume thing on the PA the other way - I've turned the volume up listening to quiet passage in a film and then a PA comes through at eardrum bursting levels. Not something that you have any control over, I understand. Another reason why avoiding the PA would be useful.

On the Change for Good thing, the envelope is always in the pack of headphones etc., so I don't think a specific PA is necessary for that, or it could be included within the welcome PA. I think that a longer PA at the start of the flight that included info like this could mean fewer interruptions later. And actually, as people are probably about to rip into their poly bags it might trigger them to do something about it, rather than ferreting about trying to find where they have stashed it when preparing the cabin for landing.