On the Event Commands window, first page, there's a Change System BGM button. You basically do this:

<> Message: "Boss: You'll never defeat me!"
<> Change System BGM: "Boss Battle Music"
<> Enemy Encounter: Boss
<> Change System BGM: "Normal Battle Music"

Make sure you do in fact reset the battle music to normal, otherwise all battles that come after that boss will have your boss music.

Attached Images

ChangeSystemBGM.PNG (19.3 KB, 18 views)