MediaGSetVolume Specifies changes to the sound volume setting. Volume values range from -1.0 to 1.0.
Negative values play no sound but preserve the absolute value of the volume setting.
The Movie Toolbox scales your media's volume in light of the track's and movie's volume settings, but it does not take into account the system speaker volume setting. This value is appropriate for use with the Sound Manager. Returns noErr if there is no error. Your derived media handler should support this function if it can play sounds. Click the link to send email to the QuickTime documentation team.