In some cases, it’s the Alexa component built-in on whatever device that the Prime Movie App is installed, that controls the App itself.
So if the Prime Movie App is installed/built-into the TV software, for example, you may find the TV manufacturer users account will ask you to agree to share information with Amazon to allow you to control the in-built TV and its Apps, but that agreement probably would not then extend to other parties, like Sonos, for example.
So my thoughts are that you may not be able to voice-control the Prime Movie App from a Sonos device for that reason.