Changing audio levels dependence if there's another video playing on top of it?

I'll try to make this question as clear as possible, if I'm not clear enough, please don't hesitate to ask me, and I'll try my best to rephrase:

I have multiple video layers in my comp, and over them I have multiple short overlying layers (all named "Bumper n" that are supposed to play from time to time while the the underlying videos continue playing. The underlying video volumes need to get a bit softer every time the overlying videos play. Is there any expression that can tell the underlying videos to play softer every time there's an overlying video layer?

Sorry if I'm not clear enough.