I have a soultion! but it will take some steps...
OK, first off, if the music set for the battle is the same as the music already playing it shouldn't start the music over again. The problem is in one line of code, but that won't stop us!
Open your project, go into the script, and find the Scene_Map section, and go to line 203 "RPG::BGM.stop"
What do you think that does? Let's comment it out (add a # to the left, it'll turn green)
Now try it, make a new event, set the battle BGM to whatever you want, then play the BGM, wait 10-20 sec, add some dialouge, whatever. Then set to call the battle, the music should not reset.
Cool? have fun, ask me if you have problems, don't forget to set the battle bgm back to normal after the battle.
(This is the first code help I've been able to provide, yay!)