On the Fire TV, when navigating down to the app tiles, the first "row" you navigate to before getting to the apps is Amazon's "Featured Content" banner. This is used to advertise new shows on Prime Video as well as paid advertisements for other streaming providers. By default, when you select this row, the content may autoplay audio and video. If you'd like to turn off the autoplay, you can do that in the Fire TV preferences.

To turn off featured content autoplay: Settings > Preferences > Featured Content

From here you can set "Allow Video Autoplay" and "Allow Audio Autoplay" to OFF

This can help save a little bit of extra data as well if you are concerned about data caps.