Didn't get past the microphone permission request. What is it needed for? It could be a good idea to first warn the user that it will be requested and how it will be used.
Also, pardon a shameless plug - not sure how your app work because of the microphone permissions, but I also wrote a freely available Mandarin listening comprehension practice tool. It's pretty simple - I downloaded all zh-*.ogg files from Wiktionary, extracted tone numbers from the filenames and asked the user what they hear. You can find it hosted here:
Source code available here:
https://github.com/d33tah/chinese-tones
Anyway, keep up the work, good Mandarin learning resources are always needed!