I EDITED MY FIRST SCRIPT WITHOUT FAILURE!!!!!! THANK YOU!!!!!!!!
EDIT: Err, right, your script ...
First, make sure it is the default RMXP script (I haven't tested it out with any CMS yet.)
Go to Line 18 of "Scene_Menu" on the script editor. It should say "def main" Hit "enter" on your keyboard and copy and paste this:
Audio.bgm_play("Audio/BGM/001-Battle01")
If you want to change what music plays, go to materials, and click BGM. Write down the name of the music file you want to play, and then type it where ---------- 001-Battle01 ------------ is on the code snippet above.
I'm working on making it stop and go back to the BGM beforehand. I'll edit it into this post once I figure it out.