Exercises without the possibility to hear sound.
It's not possible to complete the exercises without being able to hear the audio parts, where you have to translate something after playback.
Now my other computer has sound output but how would deaf people cope with this?
It was possible to do the exercises without sound. Switching off the microphone and the voice playback, the exercises which were spoken and you had to listen and then write what they said, those were written instead and you just had to translate. However, Duo on the app/phone version seems to have reverted to the original despite the settings on one's computer. The latest update did that. PLEASE change it back!
There are at least three completely different apps for the major platforms (Android, iOS, Windows Phone). E.g. the Windows Phone app still uses the old system with three hearts but the Android app has been updated to the new system where the wrong answers are repeated at the end until you got them all right once. So you really need to indicate what kind of mobile phone you are using.
I am using the Android and Windows Phone apps without sound, and I am not seeing any issues.
Also, have you looked in your apps settings? In the Android app, you can set Speaking exercises and Listening exercises to off under Settings -> Accessibility. In the Windows Phone app it's the same except it doesn't have speaking exercises anyway. Maybe your settings were reset and they are now application-specific.