The walls of the hall built for music concerts should

Step-by-step solution

The walls of a hall built for music concerts should absorb sound to prevent excessive reverberation and echoes. If the walls reflect sound, multiple reflections would cause the sound to persist for a longer time, leading to a loss of clarity in the music. Hence, sound-absorbing materials are used on the walls and ceilings of concert halls to ensure good acoustic quality. Answer: absorb sound.