Defines the percentage variation to the Volume= (0-100) value. Values out of range are ignored.
Each wave in the sound will play at a volume within that range, so a sound object with "Volume=85" with "VShift=10" will play between 80-90% absolute volume, providing some variety where only one wave is used in the sound.
Some tests indicate this may be a synthetic way to create a Doppler effect, where firing away from the viewport would be quieter than firing towards the viewport, but this isn't confirmed, and should be treated as random for now.
Note: sounds without explicit Volume= values will draw theirs from the [Defaults] section at the beginning of the sound.ini