We are currently developing a product based on the ESP32 and we are trying to get it to play with Amazon Prime Music. After contacting Amazon we got this answer:
Can you please help us with this issue ?.Hi @renovatio-systems
Just creating a new ID won't suffice. A request to an Amazon PoC needs to be sent explicitly to whitelist the ID. If you go to https://developer.amazon.com/alexa/console/avs/products -> select your product -> go to Entertainment, you'll see a list of MSPs and their respective statuses i.e. whether access has been requested/granted or not. In order to get the access you have to get in touch with Amazon's point of contact and get your product ID whitelisted.
For serial_num, it is mainly required at manufacturing time to identify individual devices. It is not required for Amazon music access. Access is specific to the product_id and all devices with the same product_id (even if they have same serial_num) will be able to access streaming services your ID has access to.
Thank you very much for your time.