The source code for Shockwave Audio is not available. SoundXtr also uses the IMoaDrSound interface to play sounds. MIX also does the file format conversion, but this is unnecessary overhead if you don't need to do file importing. The strategy that the SWA streaming asset Xtra uses to play sound is to create its own implementation of the IMoaStream interface. Remember that interfaces are just specifications.
Any class can implement an interface.
Anyway, back to the SWA streaming asset Xtra. Each of these interfaces has its own instance data, as well as the main class's instance data. You don't have to define your own interfaces in your own code, you just need some class that has an IMoaStream interface. So instead, within your Xtra's code you can invoke your own functions directly. Similarly, you don't have to have per-interface instance data, you can just have class instance data.