The annual dinner-theatre evening of the Acton Boxborough Regional High School Madrigals singing group attracted not only parents and friends of present performers and alumni who used to be in the group, but fans of the angelic voices and people who just happened to hear about the dinner. One woman in the audience was visiting from Milan, Italy. The group enjoyed a bigger venue this year at the Chelmsford Center for the Arts. Madrigal singers serenaded each table with songs such as “The Old Churchyard” and “Fair Phyllis” while lines of conscientious, black-clad high school volunteers delivered dinner to the…