"So they began shouting loudly at one another." The word "so" marks an event that happened because of something else that happened previously. In this case, the previous event is Paul's stating his belief in the resurrection.
The Pharisees are rebuking the Sadducees by affirming that spirits and angels do exist and can speak to people. Alternate translation: "Maybe a spirit or an angel has spoken with him!" (See: [[rc://en/ta/man/translate/figs-rquestion]])