Today I enabled the Alexa skill to see my newly installed Wyze Cam2. This camera is installed outside with a birdhouse cover in the front of my house. When I view it on the Show 5, I can hear the audio even though I have it turned off in the app. I cannot hear anything when viewing through the app. I have restarted the camera from the app with no change in behavior.
I believe that is how it is intended. You and/or other people in the household can hear what is going on without needing to use the app to turn the sound on.
I have 3 Echo Shows and they always play back the sound regardless of the app settings. I just say “Alexa, turn off the sound” if I don’t want to listen to outside noises.