Complex sentence: Schulz wrote operas, stage music, oratorios, and cantatas, as well as piano pieces and folk songs; he also wrote articles on music theory for Johann Georg Sulzer's (1720--1779) "Allgemeine Theorie der schönen Künste" in four volumes.
Split into simpler sentences: Schulz wrote operas, stage music, oratorios, and cantatas, as well as piano pieces and folk songs. Furthermore, he also wrote articles on music theory for Johann Georg Sulzer's (1720--1779) "Allgemeine Theorie der schönen Künste" in four volumes.