Edit: Severity increased to Major, since this bug affects multiple positional sound commands (say3D and createSoundSource).
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: