This is a discussion of the concepts of music developed by Wang Pi, Juan Chi and Chi K'ang. According to Lao Tzu, "the greatest sound [which signifies Tao, expresses the essence of the universe and] is not to be detected with mortal ears" and Wang Pi gave a precise description of this concept. Based on "the essence of the universe," Juan Chi offered significant connotations to enhance the features and functions of the traditional "aesthetically good music." As to Chi K'ang, besides giving examples to prove, in his renowned "Sheng wu aile lun," there is neither sorrow nor joy in sounds, he skillfully argued that aesthetically good music should correspond to the essence of the universe. In this study, with a view of understanding how metaphysical ideas affected their contemplation of music, I made an exhaustive search through each of their related works respectively and examined their views together with traditional music theory. In my opinion, Wang Pi, Juan Chi and Chi K'ang all believed in one fundamental principle; they all agreed that in contrast with "yu," the music which can be detected with mortal ears, there is the kind of sound which is transcendental and superb and can never be possibly detected with mortal ears. This kind of sound is "wu" and is in accord with "the essence of the universe." If music is to cultivate minds and reach the ultimate beauty of harmony, it should be in correspondence with the essence of the universe. They all claimed that, traditionally, music had long been vulgarly misused as a tool in both education and politics. Besides raising musical creation and appreciation to a much higher and sublime level, they offered creative insights and suggestions about the aesthetic quality and value of music as well. From this discussion we can understand how the metaphysical thoughts of Hsuan-hsueh can provide both constructive enlightenment and nourishment to the concept of music.