When Capulet speaks of their "solemn hyms" changing to "sullen dirges", he speaks of the rapid shift from songs of celebration to songs of sadness. Yet, his celebratory songs are not exactly full of bliss--they are "solemn"--that is consecrated and holy, but also serious. "Sullen" means gloomy, but originally it meant simply "alone". Thus, Capulet here is stating outright his sorrow at the loss of Juliet. She was his only daughter; he was expecting a wedding; what he got, however, was emptiness, lonliness, and a song of loss. It is a wonderfully poignant line, for Capulet, only a short time earlier, was willing to disown his daughter for her insubordination, yet here he reveals that when he loses her, he is alone. He loses everything.