If you are on optical, you can't use CEC to control the volume. Try with IR then. On the Sonos app, go to System Settings, choose your soundbar room, then under Home Theater be sure that Use IR is ON, click on Remote Control Setup and follow the instructions.

If you can use HDMI-ARC instead of the optical cable, then we could try something else.

Thanks for the reply but IR is already turned on as well as IR Repeater.

I don’t care about voice control, just want to be able to control the volume with the up and down volume on the FireTV remote.

What's this IR Repeater?

What's your remote?

Wouldn't you be able to connect your soundbar to HDMI-ARC instead of optical?

I've done some research. Follow these steps on your Fire TV and try adding your Sonos Arc as an equipment:

1. Go to Settings  on your Fire TV.
2. Select Equipment Control.
3. Select Manage Equipment.

   Note: Select Advanced Settings on this screen to activate or deactivate Automated Device Discovery.

4. Select Add Equipment and choose the equipment you'd like to add.

 Once the Firestick remote is paired with the Firestick used for the Arc system it should control the volume of the system.

 It will not control that volume if the TV is off.  So not for music.

How?

https://kb.nex-tech.com/article.php?id=2954

 Oddly this is all I could find on quick notice.  I’m sure there is a manual on line.

https://s3-us-west-2.amazonaws.com/customerdocumentation/Amazon+Fire+TV+User+Guides/Fire+TV+Stick+Device+Documentation/Fire+TV+Stick+4K_Quick+Start+Guide_US.pdf

The quick start guide.

 Is the Arc connected either via ARC or eARC?

That's the issue. It's on optical. So there should be a way to control using IR.

 Never tried that.  Let me know if it works.