Replacing music in Heroes III.

So, how do I do it? I know there are editors, and you can do it manually, and so on, but I don't quite understand how exactly. Not that I've been looking very hard, but still, I haven't found anything useful. I just want to add a soundtrack for the prologue to a campaign and send it to a friend. Of course, he'll probably have to add it too, but that's not the point. Is there a simple way to do this? You must have something about it somewhere on this forum.