The sound is suddenly turned off; is there a problem on the server?
In my computer, the connection to Duolingo becomes extremely slow today. I did not have any problem till yesterday night, so I guess they're doing sth about the server.
I doubt it's anything to do with the server. Audio failures have become endemic to Duolingo users (me included). Try restarting your browser and hope you can make it through at least one lesson with audio intact.