Complex sentence: Concurrently he completed an internship at Delos, where he had recorded as a boy, which led to his working with many great artists as an independent recording producer for labels such as Sony/CBS, BMG/RCA, Deutsche Grammophon, and Delos.
Split into simpler sentences: Concurrently he completed an internship at Delos, where he had recorded as a boy. This led to his working with many great artists as an independent recording producer for labels such as Sony/CBS, BMG/RCA, Deutsche Grammophon, and Delos.