Complex sentence: Theodore Spiering, who had guest conducted the orchestra, was the next appointed conductor, but was unable to begin his first season due to his untimely death in Munich (where he was searching for new scores for the orchestra).
Split into simpler sentences: Theodore Spiering, who had guest conducted the orchestra, was the next appointed conductor. Spiering was unable to begin his first season as conductor due to his untimely death in Munich (where he was searching for new scores for the orchestra).