AirPods Pro 2 connect to Amazon Kindle but no sound.

I have a new pair of AirPods Pro 2. I have an Amazon Kindle 11th Generation with up to date software. I can pair the devices, and the name that comes up on the Kindle is "Graham's AirPods Pro - Find M", which seems to refer to the Apple "Find My" feature. However when I try to play an audiobook, I get no sound, or perhaps a tiny whisper. The Kindle's audio volume is set to maximum, and I think the volume on the AirPods is up (although I don't know how to visualise the volume level).

What did I do wrong?