Wanted to see if this was possible in Storyline and IT IS!
I came across this interaction on the LA Times Entertainment section. After playing around with it for a minute or two, I wondered if it was possible to recreate it in Storyline and it is!
Here is what I did: Voice Over Interaction
The Storyline development was a piece of cake. Loading videos, setting the triggers, and creating layers was easy! The Photoshop element was a lot more time consuming. I used hotspots in order to avoid too much graphics work.
UPDATE: I reworked it for you as a template you can use.