Co-op maps help

I always play my own soundtrack.
I dislike the music of the game, and would prefer to play my own:

So I can add music by editing mission with editor, right? =D If yes then I must try it out. One more thing to ask- can I somehow add cutscenes in multiplayer for those singleplayer missions? Because I've completed half of game alone, and with my friend it would be nice to watch cutscenes. =D PS If editing map won't help have I any ohter options?(except those multi sound carts, I don't get it at all XD )