Hi, I'm Rodrigo and I will help you.
The Duolingo app on Windows runs from the Edge browser, make sure the Duolingo website has permissions to access the microphone on the Edge browser.
Paste the address below on the address bar on Microsoft Edge to go directly to the Duolingo website permissions and set the Microphone permission to "Allow"
edge://settings/privacy/sitePermissions/allSites/siteDetails?site=https://www.duolingo.com