Audio start after all button/layers have been visited and CLOSED?

i have a slide in which the viewer has to click on 7 buttons (each of which opens their own layer, each of which has it's own audio file). After the viewer clicks and closes all of those other layers, I would like an audio file to play. There's an audio file at the beginning of the slide, too. So there's an audio file with instructions, the viewer clicks through all 7 buttons and views those layers and listens to the audio on those layers, and then ideally there would be closing narration after all the buttons have been viewed.

I can get it so the audio file will play after all these 7 buttons are pushed if the viewer presses the play button again (through variables and triggers), and I can get it to play while the last of the layers is open (by putting the last audio file on it's own layer which will start when all the buttons are visited). But I want it to not start until all of the 7 button layers are closed. 

Do I just need to give up and put the closing narration on the next slide? This will get a little difficult because each button has a visited state that looks different from the starting state, so it won't be easy to fit the two slides together seamlessly enough so the viewer doesn't realize they went to a new slide.