Stereo sound sources are not particularly useful in the 3-D sound environments of DirectSound, because DirectSound creates its own stereo output from a monaural input. If an application uses stereo sound buffers, the left and right values for each sample are averaged before the 3-D processing is applied.
Applications should supply monaural sound sources when using the 3-D capabilities of DirectSound. Although the system can convert a stereo source into mono, there is no reason to supply stereo, and the conversion step wastes time.