The news just gets better and better, doesn't it?
Shortly after posting my previous comment, I experimented with changing the .dll extensions by making a copy and changing the extension on the copy. It turns out you are indeed correct, all I have to do is change the extension, and they work like a charm in XMPlay, just as they did before!
I also discovered there are more songs that aren't archived here that are in the game:
The Holiday Anthem (World 2 Map)
Ykyrococ* (Plays somewhere in World 5, I think). The star is part of the title, but I had to leave it out since Windows doesn't like that.
There's another one simply titled "End", but there are already several songs with that title, so I'm not sure if that's already on here since I'm too lazy to check every single one of them.
EDIT: I have uploaded all three that I know aren't on this archive! These were all created simply by changing the extension, and seem to work fine in XMPlay. You can find author information the samples.
Enjoy (Well, once they're no longer pending)!