A while back I was interested in getting to understand Game Boy sound engines in depth and stepped upon the name of Stello Doussis and his developments. He stood out because like other Game Boy developers at the turn of the century, he was also interested in "scene" development and involved in one of those scene2company ventures. This sound engine was originally up for lease to be used by other developers who wanted a solid music engine in their games. Stello said I could give this to anybody that I wanted as long as it was used for educational and developmental purposes.
What makes this package unique is that it contains the sound engine source for two games: Happy Hippo and Floracy. Floracy never met completion, but Stello says they have it almost complete and may sell to a virtual market. A demo of Floracy was released on their website. It looks like Pokemon with plants.
I have successfully used this source for both Happy Hippo and Floracy, but I can't find the development kit I specifically used for assembling the Happy Hippo source anymore. Once I find it or remember which one it was again, I'll post a follow-up.
There are some really neat and obscure GB music tools in here for everyones' delight. :)