Well the event is working as it should, I don't know of any way to tell Construct to wait for an external program to do something, other than waiting for JS variables or the like (which wouldn't be useful in your case anyway).
I'm not sure exactly why you're using external files, but perhaps it'd be better to instead load those wavs and pngs as project files, and call them from there, within your NW application. If all you want to do is show an image or load an audio track one after the other, then you're far better off keeping it within your application.