Virtual acoustics is a useful tool for studying ancient environments now part of our cultural heritage. The acoustic restitution of an existing building, or even of a no more existing building, allows understanding its acoustic and architectural evolution. St. John Baptistery is located in Piazza dei Miracoli, which is a homogenous architectural ensemble including the Leaning Tower and the Cathedral. Due to the succession of different masters, the Baptistery was built in stylistic disagreement with the earliest design, resulting in the actual architecture that provides interesting acoustic peculiarities. This work presents the virtual acoustic restitution of a potential first design, based on previous studies. The accurate calibration of the existing Baptistery model enables us to achieve a believable acoustics restitution through auralisation techniques. More in details the acoustic coupling between the volumes of the building – respectively the dome, the matroneum and the ambulatory – was studied with acoustic measurements and numerical simulations. Concerning the influence on absorption at low frequencies, the construction techniques used in the vault were discussed. Furthermore, the layout of a Gregorian soloists in the choir part of the Baptistery was simulated according to musicological researchers, both in unoccupied and occupied conditions.