Google Duo's audio calling feature started rolling out at the end of last month to users in Brazil after the company confirmed the option would be coming way earlier last August. Today, Googler Justin Uberti announced on Twitter that audio calls are now available worldwide to all Duo users.

I've tested on version 9.1.1516 of Google Duo, which you can find on APK Mirror or grab directly from the Play Store, and the option is live there. It didn't show up when I first opened the app, but after a few seconds the toggle was at the top allowing me to switch from video to audio call. And obviously, it works as expected.