It's useful, although a little buggy.
When I'm watching a video with accelerated speed and I activated the extension, the video gets to the normal speed. Could you fix that please?

Has a rather limited Range. I use it to have a tab play music very quietly in the background, and with the lowest sound setting on obth YT and this Addon it just about works more or less but i whish i could make it a little more quiet. A positive: the setting gets saved between different browser sessions.

It is very hard to set the right volume when the volume bar won't change the sound "live". I need to move it with the mouse, release the mouse button, listen if the volume is right. If not i have to do the same thing again and again until i find the right volume level. It would be great if it worked as a real volume button do.
You write "Fine-grained control: 0 % - 600 %" but i can only move it in step of 10.

If you install this add-on, videos from some sites cannot be played. for example, https://news.v.daum.net/v/20200512193407055

have to set it for every single video every time. I didn't even reload/refresh the browser or went into another page. if it could remember the volume level for a page at the very least before I close the tab or reloaded it.