What Tips mentioned I believe are called "transfer grills". They allow air to move between rooms but inhibit light and sound transmission.
These grills can go above a door to a room where the door is frequently closed (such as a bedroom). Or if that is not possible, they can also go in a wall, as long as the opposite side is open to the common existing return air inlets.
Keep any kind of HVAC duct out of an attic if you can help it.
Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.