Reference Manual - 1.1.1
IFusionSoundPlayback   SetDownmixLevels

Set the volume levels for downmixing.

SetDownmixLevels (
  IFusionSoundPlayback   * thiz,
  float   center,
  float   rear
);

Set the levels used for downmixing center and rear channels of a multichannel buffer (more than 2 channels).
Levels are linear factors ranging from 0.0f to 1.0f and being 0.707f (-3dB) by default.