What is the significance of the songs and music before the entrance of Caesar's ghost in Julius Caesar?

Brutus asks for music to appease his conscience because he can’t sleep, worried about Caesar and Philippi.

Brutus argues with Cassius about whether or not to go to Philippi.  Cassius, the more experienced military mind, does not want to go.  He worries that it is not a smart strategic move.  Brutus overrules him.  It turns out to that Cassius is right, and Brutus worries that he is right.  He asks Lucius to come in and play for him, unable to sleep.  His conscience is troubled because of the death of Caesar.  Brutus was convinced that killing Caesar was right.  Now, he is not so sure.  The war goes poorly.

When he asks Lucius to play for him, listen to the words he uses.

This is a sleepy tune. O murderous slumber,
Lay'st thou thy leaden mace upon my boy,
That plays thee music? Gentle knave, good night;
I will not do thee so much wrong to wake thee:
If thou dost nod, thou break'st thy instrument … (Act 4, Scene 3)

“Murderous slumber”?  It sounds like something is weighing heavily on his mind.  Brutus’s fears about Philippi and the murder of Caesar, along with the sleepy tune, form into his vision of Caesar’s ghost.  He either conjures the ghost up from his doubts and uncertainties, or he dreams him from these misgivings of not listening to Cassius about Philippi.  Caesar’s ghost appears on the tail of the music, and tells him he will see him there.

GHOST

To tell thee thou shalt see me at Philippi.

BRUTUS

Well; then I shall see thee again?

GHOST

Ay, at Philippi.

BRUTUS

Why, I will see thee at Philippi, then.(Act 4, Scene 3)

The ghost refers to himself as Brutus’s “evil spirit.”  This seems to support the idea that he is a product of Brutus’s self-doubts.  Brutus fell asleep with the music, but it was not an easy sleep.  To him, the music was a brooding, ghostly music.  Whatever song it was, it conjured up images of ghosts and visions of murder rather than comforting sleep.

Philippi was indeed uncomfortable for Brutus.  Caesar’s ghost appears several times, never giving comfort and always adding to Brutus’s self-doubts.  Since in this scene he appears after the music and song, it adds to the theory that Caesar’s ghost was a dream rather than an actual vision.  This supports the idea that Brutus’s conscience is suffering from his decisions about Caesar and the war.