Ref: https://community.bistudio.com/wiki/Arma_3:_Event_Handlers#SoundPlayed
To be able to delete the sound, if you want to make a scripted customized system.
As alternative a boolean return value at the end of the EH code execution to have the sound not be played/muted.
Thank you for requesting the feature.
Due to the current phase of Live Ops development in Arma 3, we cannot commit to any new features, though we will try to consider them.
This doesn't make sense. SoundPlayed EH doesn't fire "if you want to make a scripted customized system." by which I assume you mean playing sounds yourself.
It only plays the hardcoded engine sounds played by units. And the source object is always the unit itself that is passed in first parameter already, doesn't makse sense to pass the unit twice.