This is a wrapper in C# (1.1, what I had handy in the VM I poked the add-in in) for the strange PowerPoint custom soundtracks add-in (further discussion is on the Lobsters submission). It comes with an example application to drive the library that controls it.
Surprisingly little. The API prototypes are in the add-in's VBA-driven side, and if you open it in VBA's object browser, all the prototypes and enum values are present. Adapt to C#, and an object-oriented wrapper is obvious.
Convert to modern C# (trivial), investigate more of the API, investigate the rest of DirectMusic and its history, and consider more OO wrappers.