Sordina
The short answer: Chrome has no volume control of its own, only mute. Use the video player's slider (YouTube has one), mute noisy sites from their tab, or give Chrome as a whole its own slider with a per-app volume app.
What Chrome can do
- Mute a site: right-click its tab and choose Mute site. Every tab from that site stays muted until you unmute it.
- Mute sites by default: Chrome › Settings › Privacy and security › Site settings › Additional content settings › Sound.
- No volume level: Chrome can't set a tab or site to, say, 50%. It's on or off.
YouTube's own volume
- Use the slider in the player, or the up and down arrows to change it by 5%. M mutes.
- Stable volume, in the player's settings, evens out loud and quiet parts. It's on by default, which is why some videos sound flatter.
With Sordina
Sordina puts a slider for every app that's playing in your Mac's menu bar, so you can set Chrome against everything else in one place. That part is free.
- Click the Sordina icon in the menu bar.
- Drag Chrome's slider, or click its mute button.
- Everything else stays where it was. Chrome keeps its level until you change it.
Sordina sets Chrome as a whole, so every tab follows the slider. It can't set two tabs to different levels: for that, use each site's own player.
Common questions
One site has no sound at all
It may be muted. Right-click its tab and choose Unmute site, or check its Sound setting under Site settings.
Is there a Chrome flag for tab volume?
Not any more. Older advice points to a setting in chrome://flags that has since been removed.