The weather looks a bit iffy tomorrow, but the rain should hold off just long enough for Metropolis Ensemble's 2nd annual outdoor musical extravaganza Biophony at Brooklyn Botanic Garden, featuring the world premieres of 15 newly commissioned works. Over four hours, more than 100 musicians will perform throughout the BBG grounds, which should be teeming not only with flowers but also birds, as tomorrow is World Migratory Bird Day.