I got this guidance from Klipsch support and confirm that it works

You definitely should be able to adjust the volume on that model using the phone’s controls. This functionality does depend on an Android setting called Absolute Volume. If that setting is disabled, you won’t be able to adjust the volume using the Android device’s controls. This setting is usually enabled these days, but it’s worth double checking. To gain access to this setting you’ll first need to enable Developer Options on your phone.

Open your Settings app. Go to About Phone. Under this, look for your phone’s Build Number Once you find it, quickly tap on it seven times. You should see a message stating “You are now a developer!” Close the Settings app.

Once Developer Options has been enabled, follow the instructions below. The exact path to this section may vary depending on the model of phone, but it’ll be fairly similar.

Disconnect your Bluetooth headset/speakers. Launch the Settings app again. Go to System. Find the Developer Options and tap on it. Scroll down until you see the Networking portion. Under Networking, look for the Disable Absolute Volume setting. Toggle Disable Absolute Volume OFF to enable absolute volume control.

If you find that Absolute Volume is already enabled, or if that doesn’t do the trick, please perform a factory reset on the speaker - The Three II - Factory Reset.