Volume increases when event called multiple times

Question

So im working on a small game project, when you press the space bar the character fire a fireball calling a simple AkSoundEngine.PostEvent("Fireball", gameObject); event. Though everytime the event is called the volume gets louder every time, this can be if your holding the space bar down or pressing it sequentially. Im not sure if this is a problem with my unity script or if it is a problem with my Wwise project? Any help would be hugly appreciated.