Well, just add the Mimi Battle Music script into your project and call a li'l script in a map event:
$game_system.bgm_type = 4
so it knows to use background music type #4 (field music) from that point on.
The battle with the ghost with RED eyes shows the field music not changing into battle music, but continues through the battle and so on...
As towards the other features of the script... there's plenty and the script has LOTS of notes.
Oh, and there are two versions available:
The $global version is easier to use and configure but uses more resources. That's why it has
=begin and
=end statements around it (turning it into one big comment).
The @instance version takes a little more work, but is not hard to use. It also uses fewer resources and the demo is using it.