Could it be possible to have a section to test the microphone. For some sentences I keep getting the messages that tell me "Oops, I couldn’t understand what you said" or "I can't hear you very well". (and I'm sure my answer is correct enough :) ) It could be good to be able to set it with some example sentences/words.
There is a button that allows you to play your answer to check what it sounds like. I find that speaking slowly with a pause between the words works well, but native speakers may feel a little patronised if you get into that habit. :-)