First they hit us with a Margo Price collaboration, then a Hope Sandoval wonder. Now, Mercury Rev is back with “Okolona River Bottom Band,” the opening track from its tribute to Bobbie Gentry’s The Delta Sweete, as sung by Norah Jones. And you’ve never heard her like this before, her voice a clutch of soul stretch into psychedelic streaks. Incredible stuff. Mercury Rev has quite a few surprises in store for Big Ears 2019.