Soprano Renée Fleming, who co-runs the opera and voice program here, delivered the biggest rewards Saturday night. Demand for tickets had forced the festival to move it from the intimate confines of Harris Hall to the 2,000-seat music tent. With Inon Barnatan on piano, Fleming explored a range of beautiful music connected to nature, which dovetailed with her most recent album, “Voice of Nature,” and the festival’s theme for 2023, “Adoration of the Earth.”