Throughout history, many buildings of significant cultural value have been lost due to natural disasters (e.g., earthquakes, volcanic eruptions, subsidence, etc.) as well as other human causes (e.g., fire, war destruction, etc.). The Tajo Opera theatre was hit by an earthquake that led to the collapse of the entire structure eight months after it was built. This paper deals with the revival of the acoustic characteristics of one of the masterpieces of the architect Giovanni Carlo Sicinio Galli Bibiena. The realization of a three-dimensional (3D) model that faithfully reproduces the architectural features of the Royal Tajo Opera theatre of Lisbon allows the authors to perform acoustic simulations that reveal the sound field representing the environment perceived by the audience during artistic performances in Lisbon in 1755. In addition, the simulated results have been compared with the values of the Teatro Comunale of Bologna, which has a similar bell-shaped plan layout and has already been studied by the authors. For the comparison of the two opera theatres, both the stalls and the balconies have been considered.