How do I isolate sound between two rooms without using a door?

A couple of friends of mine wish to isolate the sound between the combined living and dining room and the bedrooms of their house. The combined living and dining area joins directly to the main corridor of the house and this main corridor runs the full length of the house to the front door. Access to the three bedrooms is through individual doors that open directly onto this main corridor. Obviously the best way would be two sets of doors with sufficient air between them at the end of the corridor nearest the combined living and dining area but THIS IS NOT AN OPTION, on aesthetic grounds. What else can be done?