Intro: Lesson 4 — 中
The lessons seem really helpful so far! I did notice that in Lesson 4 of the Introduction section, the recording for "中" is read as "naka," but it's not an option, and the answer ends up being "chuu." I know "中" can be read as both "naka" and "chuu," so that's probably where the mistake in this lesson came from, but it's probably something that should be fixed, so as not to cause any confusion for people who are learning Japanese for the first time.
Haven't gotten that far, but that's definitely something that can be confusing for new learners. I feel something like that needs a short explanation: は is pronounced as "ha," except when it's a particle, then it's pronounced as "wa." Short and simple. I don't know how this would be incorporated into Duolingo's learning system, though.
This begins to happen quite a lot as the lessons progress! It's thrown me off a few times, even though I was already familiar with some of this Japanese. I doubt there's a way to address it within the apps, but I'm hoping we'll see details about this in the web version when it's released. (I'm SO looking forward to those extra bits of learning!)