The term early music mostly refers to Medieval and Renaissance compositions, and it’s easy to imagine it as haunting choral melodies echoing off the vaulted ceilings of a stone cathedral in the 15th century. Other-worldly? Yes. Imposing? Pretty much. But this week, one of Charlottesville’s early-music ensembles, Mira, is taking a different approach to the tradition by staging a Saturday afternoon sing-in.