Can you clarify more about your connections? Is your firestick connected directly to the tv or is it connected to your avr? I am one of those that seems to have pretty good luck with CEC, so I don't know what I am doing wrong

. But I think my setup is pretty simple. I don't have a Yamaha avr. I have a Denon X4400h in use with Samsung TV.
Discounting the firestick for a moment, are you able to get sound to the AVR using any other sources connected to the tv? Do you have cable or OTA, or is firestick your only source?
I have a samsung 8000 tv from about five years ago, and it comes with one of the bluetooth universal remotes (without all of the buttons) and there is a remote setup function on the tv that allows a specific device to be added to the tv in a known fashion. That seems to be so the proper remote commands can be employed. With the tv new and CEC turned on, I simply connected the avr via hdmi to the ARC HDMI port on the tv and powered on the AVR. When I did, the Samsung TV automatically recognized the device and the model and now when I look at the TIZEN control bar (the menu bar at the botom of the screen) I actually have an icon that appears for the Denon. When I select it, I can get all of the on-screen menus and controls. IF you have a source beyond the firestick, I would be curious to know if you get any sound through your dvr? But it seems to me that if the tv can recognize your avr, that would be a good start.
Also note that I connected my Marantz 5010 to the same tv. This time, it recognized a connection, but it could not identify the specific avr model. I was presented with an input field to select the device type (avr), brand (Denon) and model, and I ended up typing the 5010 manually since it was not in the list by default and if I recall correctly, it made me search, and it did find it. Maybe you would have similar luck with the Yamaha.
I don't know if all of these steps would or should be normally required, especially since you have a different avr, but maybe there is something here that can spark an idea or point to a menu on your tv which would take you down another path to try.