Please help spread the word that when there is no sound, this is a Duolingo issue. There is nothing that the forum participants or even the Esperanto course team can do about it. Support supposedly knows, but it may be helpful for learners to continue to file bug reports so that they know it continues to be an issue and that people care about this.
To file a bug report, go to support.duolingo.com
To hear Esperanto go to Esperanto Variety Show on YouTube.