Reference Manual - 1.1.1
IFusionSound   SetMasterVolume

Set master volume level.

SetMasterVolume (
  IFusionSound   * thiz,
  float   level
);

Set the master volume level (i.e. that applies to all playbacks).
The level is a linear factor ranging from 0.0f to 1.0f.

See also GetMasterVolume().