I have just tried music.youtube.com and setting a negative db with the slider is working at lowering the volume as intended. Also typing a negative value worked for lowering the volume. Mono is also working.
https://1drv.ms/u/s!Ao1P9fFGIyXKotR7OXx1x-u9iZ3vvg?e=lNdQGd
Are you using any other extensions? Please elaborate and feel free to contact me on the github or emailing chaython@live.ca.
I'd love to help you troubleshoot your complication.
Thanks, look forward to your contact, or you finding the complication and the review revised to a beautiful 5 star :D