A Boston Symphony Orchestra audience had to evacuate Symphony Hall last night after a fire alarm went off inside. Bernadette Horgan, a spokeswoman for the orchestra, said the evacuation occurred at about 9:50 p.m., about 10 minutes before the end of the concert at the Massachusetts Avenue venue. She said there did not appear to be a fire risk, and staff was awaiting more information from the Fire Department last night. Steve MacDonald, a Fire Department spokesman, said that the alarm was false and that firefighters left after a few minutes. They did not order the evacuation, MacDonald said. It was not immediately clear last night why the alarm went off.