The lesson sounds work, but the sounds signifying correct/not correct answers have stopped working suddenly. Also, the sounds at the end of a lesson indicating success no longer work. I think there's something obvious going on, but I can't find it. Thanks!
UPDATE: If you have the AdBlock Plus in your Firefox Add-ons do the following: While you have the Duolingo website up, look on the button bar at the top of your browser for the red ABP stop sign button. Click on it and to bring down the menu, then click on "Disable on duolingo.com". The ABP stop sign should turn gray. Now refresh your page. This fixed my problem. Hope it works for you, too.