FusionSound Reference Manual
IFusionSoundMusicProvider   PlayToBuffer

Play selected track rendering it into the destination buffer.

PlayToBuffer (
  IFusionSoundMusicProvider   * thiz,
  IFusionSoundBuffer   * destination,
  FMBufferCallback   callback,
  void   * ctx
);

Optionally a callback can be registered that is called after each buffer write.
The callback is passed the number of samples per channels actually written to the destination buffer.