I looked in stackoverflow and since this is an experimental technology, there is not much talk about it anywhere on the internet.
The story I created has several youtube videos as backgrounds, in fill grid layers. When the first one starts, it never stops, overlapping the audio with the next, and so on.
The thing is, I can't reproduce the issue on my work computer and neither on my phone (tested with chrome and firefox on both devices) because I can't find the audio button to turn it on. And it seems that there is no way to turn it on by default.
Now I'm not even sure this button ever existed, but I remember seeing it 15 days ago when I created my first amp story. EDIT: I saw the button on another computer with chrome 64. I have version 65.
amp-youtube is not yet supported as a child of amp-story-grid-layer. The full list of supported children can be found in the amp-story-grid-layer validation spec.
The audio control is currently only present on stories that have audio through one of the supported mechanisms (an amp-audio/amp-video tag, or the background-audio attribute).