Can no longer hear audio on my phone app

I received my track E recorder and connected it to the app and was able to hear the audio using headphones connected to my phone. But when I set it up again the next day, I could see the audio visually being registered, but could not hear it. I tried with and without headphones and several levels of phone volume as well as using the speaker button, to no avail. I went ahead and recorded anyways and it did capture audio – but I would like to be able to hear the levels on my app again. What might be wrong?