No sound after update
I'm on Firefox on a Lenovo T450 Thinkpad running Windows 10 with all the latest updates. Until today, no problem with sound. As of today, no sound on DuoLingo, but I have good sound on YouTube and other sites. I also have good sound on stand-alone Skype.
Windows had a major update last evening, and Firefox had what appeared to be a fairly big update a day or so ago. I don't know which prompted this sound trouble.
Have you tried opening the site in other browsers? I am able to use DuoLingo in both Firefox and Chrome; if you are able to use it in Chrome or some other browser this can help narrow down where the problem lies...it might be something about the particular update of Firefox that broke, but it could also be the Windows update.
Also, narrowing it down like this could help make a bug report more useful to the DuoLingo team if you report it.