I think the issue here is that Kodi has two different methods of controlling volume and the situation here is that the internal volume got adjusted down but the volume control is currently controlling CEC and not the internal volume so it is stuck at a low level. To elaborate, sans CEC amplifier type device, when you use the volume control on a remote or keyboard Kodi adjusts the signal level it is sending out. So in a basic type setup with just a TV, a Kodi box, and a rf remote, the TV’s volume would get turned up and not adjusted and the volume buttons on the remote would control how loud the volume is being sent to the TV. If you connect a soundbar, AVR, or other device that has CEC enabled and reports itself as an amplifier type device then Kodi will shift its volume control from adjusting the internal volume to sending out CEC volume commands to that device. If the internal volume was low you are a bit stuck since the remote no longer has the ability to adjust it back to 100% which is where you would want it when using an amplifier.
A common way to fix this is often to use a Kodi remote app on a phone or tablet, or the web interface from a PC and adjust the volume there. Most apps, and AFAIK all the web interfaces will only modify the internal volume level.
There is also an option to unplug your soundbar/AVR long enough to adjust your volume back up before reconnecting it. You may have to do a power cycle of your equipment if Kodi isn’t picking up hardware change.
You could also ssh into your device and run the following command…
kodi-send -a "SetVolume(100,showvolumebar)"