"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 use this question to suggest that Paul may have gotten his teaching from an angel or a spirit. They may be rebuking the Sadducees for not believing that spirits and angels 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]])