“The Demon Barber of Fleet Street has come to rank with Wozzeck and Peter Grimes in opera’s catalogue of 20th-century tragic heroes,” writes Rupert Christiansen. And thus “Sweeney Todd” comes to the Royal Opera House at Covent Garden. The dialogue is being miked…er, make that “reinforced.” Why? Audiences are lazy today, says the production’s conductor. “It’s very destructive. I recognise that synthesizers are useful, but they eliminate instruments like the harp or guitar, and we’ve lost the possibilities of creating a deep string sound. It’s a joy at Covent Garden to be playing Sweeney Todd with a 50-piece orchestra.”