Preventing "Telephoning" Through Heating Ducts

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We have a ranch house with forced air heating and cooling.  The master bedroom is across the hall from the nursery and the ductwork runs under the floor between the rooms.  With both doors closed, we can hear the baby in his room though the vents. Is there a way to prevent the sound transmistion short of covering up the ducts with blankets? Covering the ducts will make the ineffective for heating/cooling.