I've begun using the Yanfly battle engine demo for my game but I have a problem, when I go into system to change the battle music to my own mp3 and I test the game whenever I get into a random battle rather than playing the music I told it too, it instead plays one of the default battle themes. (It's random which one plays.) I can't find a way to get it to play it, it works fine on the vanilla game but not on yanfly. Can anyone help?

Did you remember to save the game? A common mistake I've made before when editing a script is not saving before testing out one thing or another in the Troops - Battle Test.

Alternatively, it sounds like you're trying to get two scripts to work together. If you're using another script to randomize BGM for every battle start then it could be conflicting. Melody doesn't work well with other scripts that affect the battle system.

By saving do you mean clicking Apply? Since I made sure to do that.
The BGM just isn't changing at all for random battles, it'll play what I want for stationary battles (like bosses) when I put an event command to play it, but still it will play one of the default battle tunes for random encounter even though I changed it.
And what I mean by random is that it will only play a random default battle theme, and the imported one I chose in the database.
If you have played the demo map fro YEM it will play a random default theme for battles, it's like that and it won't let me change it. I have serched Google loads of times but I can't even find someone with the same problem.

No, whenever you make changes to a script you must save the project (By clicking the blue floppy disc or however you prefer). Clicking apply or save just in the script editing portion won't have any effect on your game/gameplay until you save the game itself. Just something to keep in mind for a future reference as well.

Now then, when you're adding in your MP3 for your battle music. You're applying it in the default editor under System - Battle: Right?

If so, once you place your MP3 here you also want to edit the script -YEM Party Influenced Music- and either delete this script or make the necessary changes.

It worked! Thanks a lot. I can continue with my game now.
EDIT: I have another issue it's not too big, but it's a bit annoying. It's the "Super Skills" you have (Like "Limit" "Smash" etc.) whenever I have an Actor as a class the default characters were they still have it even at level 1, it's really overpowered. Is there anyway too change the level they get this skill?