sound and flash blocker
When I first started using Duolingo, the sound worked perfectly, but suddenly a box now appears saying that "duolingo cannot play sound" and either refresh (which I do with no results) or add duolingo to the white list in your flash blocker. How do I access that? (A search of my files did not help.)
Click on Tools-Add-ons-Extensions. Look and see if you have Flashblock installed. If so click on "options" by it then click on "whitelist" and add duolingo.com to the list. If you do not have flashblock see if you have "NoScript" installed. If so you may have to add duolingo.com to its whitelist.