just look in scene map for this:
 def call_battle
  @spriteset.update
  Graphics.update
  $game_player.make_encounter_count
  $game_player.straighten
  $game_temp.map_bgm = RPG::BGM.last
  $game_temp.map_bgs = RPG::BGS.last
  RPG::BGM.stop
  RPG::BGS.stop
  Sound.play_battle_start
  $game_system.battle_bgm.play
  $game_temp.next_scene = nil
  $scene = Scene_Battle.new
 end
Â
and remove these lines
   RPG::BGM.stop
  RPG::BGS.stop
Then all you must do is to set the battle bgm to the same as the map bgm via events.
That way you can set differents musics for bosses if you need.