No, there's definitely more to it than that. If you play, for example, Battle02 from the RTP with the Sound Player, it only plays the intro once. I understand that it may automatically have the end of the song as the loop point, but once it does get to the end of the song, it doesn't play the intro again. It doesn't end and then restart. And it's not just that the RTP songs have something special about them that would only work for those specific default songs. Because if I export it and re-import it, (so that it has a red circle, meaning it's a custom song) It still loops. Custom songs that I download that were made for XP to use also have proper loop points.
Well, there's something that I thought that I said earlier, but apparently I didn't so here it is: I want to clarify about what I want. I make a battle song, and it has an definite beginning, that maybe starts with only a few instruments or slowly to build up suspense and tension, but then it goes into a much faster paced, more "epic" part, that it continues throughout the entire battle. So when XP gets to the end of the song, I don't want it to go back and play the slow part in the middle of an intense battle. Even many of the RTP songs do this. (P.S. I made no such song nor have any such song, I'm nowhere near that good at making music.)