Fundamentals of Audio and Video Programming for Games (Pro-Developer)

The MultiAudio DLL is a DirectShow filter that multiplexes one or more audio streams into a single multichannel audio stream. Although the MCAudio tool uses a fixed set of channels for 5.1 and 6.1 audio, the filter supports all 18 channels that are defined for the WAVEFORMATEXTENSIBLE structure, so you could easily convert the application to support more channels.

The Wavdest DLL is a DirectShow filter for writing .wav files. This filter is provided as an SDK sample in the DirectX SDK. The source code can be found in the Samples\C++\DirectShow\Filters\WavDest directory.