Homepod 2 volume won't decrease all the way

• first try to restart your Apple TV and Homepod - can be done with the Home app. If you don't see an option to restart, or HomePod still doesn't respond after restarting, reset it to factory settings. You can other wise just unplug them for 10 seconds to Restart.
• When HomePods are set up as a stereo pair, you must ungroup the stereo pair before you can reset HomePod.
• Reset each one by using the pressing the top method: Reset HomePod or HomePod mini - Apple Support
• After you reset HomePod, set up HomePod again.

Before going through a reset & set up, you might want to wait for the next software release , coming shorty (might be next week). After updating the Apple TV, iOS device (used to set them up) and HomePods it may work.

You also might want to ensure your Siri Remote is charged. Also see: If your Siri Remote or Apple TV Remote isn't working - Apple Support

Sounds (!) like you have encountered the stereo Homepod issue - one of the Homepods fails to respond to volume control:

https://discussions.apple.com/thread/255077557

The sequence that I list at the end of the discussion has worked for me (happens nearly every day - I suspect when my Apple TV goes to sleep).

Apparently Apple is aware of the problem so a future OS update might fix it.

I’d try the restarts on everything first so you can avoid the resets. Restarting Apple TV can often address the remote not working. I think these software updates are September 18.