the man that hath no music in himself,

Nor is not moved with concord of sweet sounds,

Is fit for treasons, stratagems, and spoils.

The motions of his spirit are dull as night

Hear you me, Jessica,
Lock up my doors; and when you hear the drum...stop my house's ears

Shylock's refusal to hear music reflects the way he feels under assault by the Christian world, but on a deeper level, it reveals a materialistic nature that, as Lorenzo puts it, is focused on "stratagems" and "spoils." Shylock shows himself as focusing his attention on the crasser side of life.

Those who can hear and appreciate the music of life, as Bassanio can when he chooses the correct casket, are open to the fulness of all of the world's rewards. He is a contrast to other suitors, who are too focused on what they hope to gain from marriage with Portia to hear the "music" of marriage. They cannot, as Lorenzo puts it,

sit and let the sounds of music
Creep in our ears

Shylock's being shut off from the sounds of music foreshadows the way he will insist on the pound of Antonio's flesh. He is shown as legalistic and unable to hear the "music" of Portia words as she pleads for mercy. However, Shylock is not treated with much "music" of mercy either after the tables turn.