Force certain files to be placed in obb

Hello every1.
I have started making a Guitar Hero styled game. I’ve got some stuff working, but I am not familiar using expansion files yet.
I am trying to force Unity3d to put the mp3 files somehow in an obb expansion file, however I do not know how to do this. It keeps putting them inside the APK and it creates a 9kb obb file…
Is there any help file I can refer to or can some1 tell me how to do this?

Are the MP3 audios directly in the scene files, or do you dynamically load them from e.g. Resources folder?