I have found that lernu uses Flash to play sound. My browser can't run Flash so I've written a user script to make lernu use HTML 5 Audio instead.

I thought it may be useful if you're, like me, using Firefox going through the first course and unable to hear the pronunciation when you click the speaker icon. You'll need Firefox with GreaseMonkey, then you can install the script from here:

https://openuserjs.org/scripts/thomper/Lernu_Engli...

This might have already been solved by someone, but the technical forum is in Esperanto so I can't read it.