When I cast music from my Amazon Music app to my CCA the volume is attenuated. When I connect to the CCA to cast the notification sound is at the correct volume level. Also, when I select "Cast my audio* and play from the Amazon Music app (or anything else) the volume is correct.
This problem was identified a few years ago (using Spotify). It was eventually resolved with some backend changes.
Hey there TheRealTomMac,
I apologize for the late response, thank you for being patient! Were you able to fix that Amazon Music issue? If not, I'd be happy to help out, as I can imagine this issue is a bit annoying. The first thing I would check is if your Google Home and your Amazon Music app are both up-to-date. Then, I would clear out the cached app data on both apps as well. Finally, to see if it helps, go into the Google Home app and disable surround sound.
I look forward to your response, and if you have any other questions or concerns for me, please let me know!