Rejoice: live music is back. Or at least, live music with a live audience, which, as Sir Simon Rattle admitted, addressing the masked and socially distanced crowd immediately before the LSO’s first full-scale public performance for 14 months, is kind of the whole point. Yes, he said, they’d streamed online concerts from the Barbican, but the silence of emptiness is a very different proposition from the silence of a hall containing 1,000 human beings.