This album is titled “Sir!” because it is dedicated to the famous Concerto for Cello and Orchestra by the English composer Sir Edward Elgar. But also because Astrig Siranossian plays Sir John Barbirolli’s cello, a magnificent instrument by Gennaro Gagliano from 1756, on loan from the Henrot Fund for the Promotion of Young French Chamber Musicians. Barbirolli was a passionate advocate of Elgar’s music, but also of Bruch’s, whose Kol Nidrei Astrig has recorded—“one of the most beautiful works ever written for the cello,” says the cellist, who is accompanied by the London Symphony Orchestra under the baton of François-Xavier Roth. Also on the program, this time with her friend Nathanaël Gouin at the piano, is the sonata by Britten, who declined the title of “Sir” but was a Lord…