Sets the relative volume of the background music. This would be a multiplier to the user's snd_musicvolume setting: 1.0 to the function is considered normal volume.
FadeTime: How many seconds to fade the music across. Use 0 (the default) to instantly change the volume.
FadeType: Linear or Logarithmic. (Not vital, choose the best option and use that.)
The song can be gracefully faded to silence before a special event, or quieted in certain situations (E.G. character dialog).
LocalSetMusicVolume would affect only the activator.