I haven't had a chance to look at your app, but it sounds like a great idea! A while back I did some language teaching my self. There have been some studies done to understand how the brain works in regards to language. You may want to see if you can get your hands on some of that. Maybe you have already.
But any way, one thing I remember reading is how one can increase the alertness of the brain in order to improve the way it listens to the nuances of pronunciation. Alertness can be improved by providing a mechanical, or coordination, challenge to the brain at the moment of listening to the pronounced word(s). So, for example, you could provide some quick moving ball that the user needs to try to click while listening to the words, or something like that. Just a thought.