I just turn off the audio questions. I'm not going to waste my time on readers who mumble and leave me to guess what they're saying. My problem with Duolingo isn't that they are not perfect. Even I am not perfect. The problem is that obvious shortcomings go on year after year and they are not corrected.
I've already said this to you in a different way, so this is for the benefit of those reading along...
Using the "can't listen now" option is a great suggestion if you have that much problem with the audio in the course. I've even added your suggestion to my "Too Slow, Too Fast, Not Clear" post (see link). On the other hand, it strikes me as exceedingly arrogant to go from "I don't understand" to "this course has readers who mumble." As someone who actually speaks Esperanto, I can tell you that the readers certainly are not mumbling. If it seems to you that they are, then this only means that you still have more to learn -- which makes perfect sense if you're here taking a course to learn Esperanto.
To those who do choose to turn off the audio, I hope you find other ways to train your ear to the sounds of Esperanto -- because this is the only way you will learn to actually use the spoken language.