close your vents in the summer and open them in the winter

We opted to keep it closed up for security. We lock the door to basement at night (and motion sensors) and if someone gets in from the back, we still have another barrier...

I would be worried about sound. For us the basement is an "away space" where the teenagers can turn up music and be loud.

We have this. It actually helps circulation, and new homes have really good insulation anyway.

I noticed having an open stairway to a finished lower level (basement) is pretty popular in new homes being built. What would this do in term of heating efficiency? Wouldn't a colder lower level be even harder to heat if all the warm air is free to rise to the main level? Or are basements so well insulated now that it isn't an issue?