Please elaborate. What is the issue? What sites does the issue persist on? Please leave constructive feedback, so I can assist you and the community.

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

Major update, required new permissions should work on basically every site now.
Do you recall what site you were on? Yes it specifies it only works with HTML audio, this is to require less permissions.

Do you recall what site you were on? Yes it specifies it only works with HTML audio, this is to require less permissions.

Do you recall what site you were on? Yes it specifies it only works with HTML audio, this is to require less permissions.

Do you recall what site you were on? Yes it specifies it only works with HTML audio, this is to require less permissions.

As stated in the description, it only works on HTML5 video, which site did you try to change the volume on? It works but some sites have CSP rules that block it from initializing/breaks audio. Add those sites to the blocklist in the settings. More and more sites have been adding CSP rules to block extension manipulation to fight off exploits and bots...