I once sat in a rehearsal room in a brick-box theatre on the outskirts of Cape Town. The cast was warming up for Carmen. First, the choreographer put 40 mostly black South African singers through a gruelling physical warm-up. Opera singers are rarely slender, and they were all in a muck sweat by the time the vocal coach stepped forward to lead them through a vocal warm-up. But when they opened their mouths it was as if someone has strapped you to a chair in a wind tunnel. The noise was transforming, majestic, all-powerful. So I knew roughly what sound to expect in Singing for Life, a documentary about the miscegenation of the black township choral tradition and the white man’s most exclusive art form, opera.