I believe you can cast to only one Sonos speaker from the Amazon Music app. If you want to cast to a group, the speakers must already be grouped in the Sonos app before you cast from the Amazon Music app.

That is what I observe with the Amazon Music app but it’s not as flexible as what the Spotify app does.

SPOTIFY:  I can change the speaker groups in the Sonos app on Android and if I keep the Spotify app open it will show the changes I make seconds later. That’s really great!

AMAZON MUSIC: On the other hand, the Amazon Music app can’t see the Spotify groups at all. As you mention, Amazon can identify and play on one speaker of an existing Sonos group and then the rest of the speakers in the Sonos group will also play. That’s much less good.

I was wondering if there is a way to make Amazon Music aas flexible as Spotify.

Amazon Music would need to more fully implement the available API that Sonos provides. This would be the same API that Spotify uses. 

Unfortunately, this puts the work back on Amazon, not Sonos, so it would depend on Amazon engineers and time.