kplhardison is right in identifying music as a key element of Twelfth Night - much more than his other comedies. In fact, music permeates so much of the action, capturing the theme of love in the play and exploring it in different ways. You might want to think about the role of Feste in this sense - many productions have him singing to the Duke as the play begins. We know that he moves between the houses, and much of the music in the play is sung by him. Also, he is given the "final word" of the play with a rather puzzling song that talks about the course of life much as Jacques´speech in As You Like It talks about the seven stages of life. This adds a much more melancholy element to the "happy ending" of the play, as we are left with the "wind and the rain" of life to contend with. So think about how music is also used to undercut the comedy in this play.
Twelfth Night opens with one of the greatest entrance lines ever written: "If music be the food of love, play on." This sets one of the themes for Shakespeare's romantic comedy Twelfth Night, that of music as the inciter (food) of love, which can be well played to success or badly played to failure. The following instances are where Shakespeare makes his theme clear.
1) Viola goes to Duke Orsino disguised as Cesario because she is confident that she can sing in many sorts of music (Act I).
2) Sir Andrew's ability on the viol-de-gamboys fails to impress Maria (Act i).
3) Sir Toby requests that Feste sing a song, and Feste chooses a song about the people whom a person loves (Act II).
4) Olivia tells Cesario that if "he" were to sing "his" own love suit to her that that song would be sweeter than the music of heaven (Act III).
Tellingly, Act IV, the setting for Sebastian's conflicts and for Malvolio's conflicts--acts of malice instead of love--has no music, singing or references to music or singing.
5) Olivia tells Orsino that if he is going to beg her to marry him again ("the old tune"), it will sound to her like the howling of a dog after having listened to the sweet strains of music (Act V).
6) In Feste's final song that closes the play, he says that swaggering (boasting and bragging [like Orsino and Malvolio both]) never gets a wife (Act V).
In these instances, Shakespeare ties music and song to love and and makes a distinct division between those who sing the song of love well and those who do it badly. It is important to note that there is not a clause added to the theme that stipulates that singing the song of love well or badly depends on the perception of the person being addressed. Doing it well or badly is shown by Shakespeare to stand in accord with the suitor's behavior and, in Sir Andrew's case, choice of instrument.