createSoundSource sound volume is **permanently affected** by being inside a vehicle (in 1st person camera view) at the time of creation. The volume appears to be permanently and significantly reduced. If the command is used when the player is outside a vehicle or when the player is inside the vehicle and in 3d person camera view, the volume appears to be normal and remains normal regardless of future behaviour.
I believe the sound reduction coef when inside the vehicle may be being incorrectly permanently applied to the sound volume of the effect.
I found a person who discovered the same issue in 2016:
https://forums.bohemia.net/forums/topic/189451-createsoundsource-volume-differs-by-crew-position/