Sound proofing bedroom doors, what to do about air flow?
Anytime you replace a soft flooring (carpet) with a hard flooring (hardwood) you get a noisy footstep. Isolating and soundproofing the bedrooms from the hallway is nearly impossible. If you have an older house, with supply and return ducting in that room, you can replace the doors with solid core doors and a reduced bottom undercut to help reduce the noise transmission. With a newer home that only has a central return duct, usually in a hallway, you can not reduce the doors undercut since this the return air from the room. The central return allows a HVAC company to save a few bucks at the home owners expense. To me, this is a crappy and cheap job. I got into the HVAC business in the late 1960's and this type of installation was unheard of. Today, almost every contractor is doing it. 20+ years ago, when I built this house all the contractors that wanted to install a central return were given their walking papers. I searched until found a contractor that would install an older type duct system. I do not know what you can do at the bedroom locations except recommend walking in your socks and leave slippers and shoes for the other areas. You could insulate the bedroom walls and install solid cor doors to reduce some of the noise but the solid surfaces are still going to be noisy.  my 2 cents