Did you check your settings on Duolingo and make sure that "Speaker" and "Voice Autplay" is on?

yes everything is turned on

for other sites my sound is heard, but in duolingo it does not work

• try reloading the website (press F5)
• try another browser (i.e. if you have Chrome try Firefox and vice versa)

That's happened to me a lot before, you could try either refresh the page or log out and login again, if that doesn't work try using chrome browser if you aren't already :)

(I was using safari browser when I had this problem, after switching to chrome it's been fine :) )