1. Open the Alexa app on your smartphone or tablet
2. Go to "Devices > All" and see if you see your TV listed.
3. You can tap the device, then the Settings wheel, followed by tapping the name to change it, therefore you would ask your Fire TV Cube "Alexa, mute NAME OF TV".
If you are running audio through a soundbar, you would use the command to mute the name of the soundbar.