I am having the same problem as Anna. In mi case, the sound stopped working when I was in the middle of the flashcards. I mean, let's say I have 100 words or phrases, then when I was studying the number 50 the sound stopped, and after that, none of the following words or phrases have sound. I use Firefox and Chrome, but I didn't have this problem before with any lesson. I am studying "#301 Steve and Alex - Multilingualism (Part 1)"
Thank you for your help!