If you can't hear the audio, it's possible that the app was unable to load the sound files from our servers.
Please, make sure you have a good internet connection and that you have enabled the Sound Effects option within Settings.
Once you’ve verified your internet connection, please close the Drops app and then open it again.
The audio files should start downloading automatically.
If you are located in China, kindly consider using a VPN.
If that didn't help, please reinstall the app on the same device you have learned your words so far.