Malbim: Sometimes melancholy is due to a loss of external (physical) Kelim, or the brain or arteries. Sometimes it is due to a Dimyoni (imagined) loss. After Ru'ach Hashem left him, he was unlike others. He was always worried, about his punishment and loss of kingship due to his sin, and through this his blood 'burned', and melancholy comes from this and red bile. Sometimes via Hashgachah, Ru'ach of Dimyon comes on his soul, and he is constantly agitated and terrified. His cure is via external cures to fix the Kelim, or an inner reason to straighten the Dimyon and thoughts to Simchah and serenity. His servants thought that music can uproot the soul from nature to nature, from sadness to Simchah.