what is the meaning of "If he, compact of jars, grow musical, We shall have shortly discord in the spheres....?"

The Duk believes that of this conflicted man has acquired a taste for music there has to be somethingthing desperately wrong.

Duke Senior says that 'If he' which is Jaques is made up of discords or unhappiness which is a good description of Jacques sarcastic temperament, which makes him clash with ordinary people. Grow Musical here is a reference to the Medieval Astronomy, the planets or spheres here, produced harmonious sounds which is referred to music here, as they revolved. The music sounds were regarded as the music of the planets by Duke Senior. He says that this music will become discordant before Jaques acquired a musical sense.