The amiable Oriana Singers, with their nicely schooled voices, are not squeamish about experimenting with new ways of presenting their rather extensive repertoire. They clearly impressed the judges at an international vocal-ensemble competition in Stuttgart in June, for they were awarded a well-deserved prize. They are now back and giving two free concerts, sponsored by the Park District, in which they share a long program with noted storyteller and Autoharpist Beth Horner. Among the singer's eclectic selections are German and French Renaissance madrigals about birds and two songs by the first important American-born composer, William Billings. Saturday, 10:30 AM, Indian Boundary Park, 2500 W. Lunt. Sunday, 3 PM, Douglas Park, 14th and Albany; 262-4558.