Sounds like you have an issue muting the audio on the Echo, but everyone should be aware the audio on each camera is live all the time. You can mute it in the app for a single device, but the audio is always being transmitted by the camera. You should especially be aware of this if you are sharing cams with other people.
Note you can turn the recording of the sound to the SD card off, but this does not stop the camera from transmitting the audio in the live stream.
The only way to assure you stop the transmission of audio (and maybe get around your mute problem on the Echo) is to physically remove the microphone on each camera:
And here is a wishlist item to VOTE for control over the audio in the live stream:
Here is a wishlist item to VOTE for control over user permissions (you wouldn’t want to turn off the live stream’s audio, only to have the person you are sharing the stream with turn it back on!)