The character ha (は)
Just a big heads up. When the character は is in the position of being the topic/subject marking particle it is not pronounced "ha" but rather "wa". This is a big deal. The Duolingo voice seems to consistently pronounce it as "ha". This isn't just annoying being such a basic thing but would be completely misleading to beginners who will be practicing the wrong thing over and over. Not sure how this could have gotten out the door this way.
Yes, there should be tips and notes on the app—I can't think of a better way to explain things like this.
I also wonder why there is no slow-speed button for Japanese—all the other TTS-voice courses have it, and there's nowhere where it would be more useful than in this infamously quickly-spoken language.
And there's a huge audio problem when it comes to onyomi / kunyomi, and counters/numbers, which I haven't even studied, so I'm having a chaotic experience finding out what should be pronounced like what and when...
( I don't know if it's called onyomi/kunyomi for numbers, but you get it, the different pronounciations)
I have yet to start the course (waiting for the web version), so I can't comment on my own experience, but that sounds worrying. It's only for the counters/numbers, though, at least? I guess for other things usually there is either only one obvious/most common way to say it (as with vocabulary) or the reading to be chosen may be understood from context (although I wouldn't know how well a TTS system would be able to pick up on said context in that sense).
I think it is pronounced correctly in the complete Japanese sentences. However, in the exercises where you translate an English sentence by arranging different Japanese words and syllables, it's only pronounced "ha", even when it functions as a topic marker and thus should be "wa".
A similar problem I noticed is that in sentences where the word "ha" is used to mean "tooth/teeth," the speaker pronounces it like the "wa" particle, when in that case it should be pronounced as "ha."
I also noticed the on'yomi/kun'yomi pronunciation problem. For this, I think it would be a good idea to include the furigana with the kanji and have them as two separate answer options.