The way I read you post, the 4" x 8" 48" OC joists merely support a new T&G ceiling and soundproofing, have no other load bearing requirement (storage or occupancy). You don't provide the span distance. Regardless of span, the joists are almost decorative, because they only support the ceiling. All roof loads continue to carry through the existing rafter systems. If my understanding is correct, you do not need a center beam to carry the joists. If the area above the joists is ever intended for storage or occupancy, the answer changes.