Let me congratulate your teacher on this innovative assignment! Capturing the themes of Macbeth in musical form will greatly enlighten you on the play's textures and rhythms. First, I would warn against any obvious "witch-like" music, just as the costumer should avoid pointed hats, etc. The essence of Act I is the way that human actions (war, promotion in rank, etc.) contrast with the "other-worldly" activities of spell-casting, calling on higher powers, and (self-fulfilling) prophecy. So the music should try to demonstrate this contrast. Second, the other driving force in Act I is ambition vs. fate--to what degree does human will/choice overcome the pre-ordained actions of the universe, if at all? As to what musical vocabulary will accomplish these ends, I suggest dissonance ("Fair is foul, and foul is fair"); close attention to the balance of predictability vs. surprise in melody, orchestation, and key signatures; and an undertone that emphasizes the darkness of the consequences of this contrast as events unfold--something between Carl Orff and Arvo Part?