If on PC, go to your name on the top, click Settings, then you can enable sound effects and speaker if you wish. If you're on mobile, I don't know. It could just be your device. Make sure it's not muted.
It's worth looking in the help section of the website for these sorts of questions. There's a link at the bottom of every Duolingo page, and in the menu that appears when you hold your mouse over the username at the top of the page.
Have a look at this article from the help section: https://support.duolingo.com/hc/en-us/articles/204882510-My-sound-is-not-working-What-can-I-do-