When will be the audio exercises available on Duolingo in Esperanto course?
Well, I have had not so good experiences with audio exercises in French and German. There were times when I did the audio excercises when I would speak the sentence, but it let me move on, even though I was sure I had said it wrong. In my opinion, if you can't have a good pronunciation feedback system in your natural-made languages, then you should definitely not have one in Esperanto, as its pronounciation is unique because it combines the phonics of many different already-existing languages.
They just started working yesterday. Actually I was on a plane that had a short stopover (I didn't even get out of the plane). I turned on the data on my phone to do an exercise and then suddenly there was a listening exercise. I was using the thing without sound so I had to hunt for my earphones :)