There are a few reasons this could happen. Here are some things to check:

• Make sure the correct microphone and speakers are selected in the desktop app. To check this, click Settings at the top right of the app and go to Audio Settings.
• If this still doesn't work, open your browser settings to make sure that your microphone and speakers are being detected. For Chrome users, copy and paste the follow link in your chrome browser chrome://settings/content/microphone?search=microphone