Reverberation produced in a large auditorium is due to: 

• Just like light reflects off a mirror, sound waves reflect off any flat hard surface.
• Reverberation is the persistence of sound after it has been stopped due to multiple reflections from surfaces like walls, ceilings, floors, etc.
• In a large auditorium, sound gets reflected multiple times from the walls, ceilings and floors causing reverberation.

Therefore, we can say that reverberation produced in a large Auditorium is due to the reflection of a sound by walls and ceiling.