Right now, if you left-click on a tab, you'll see "Mute tab" listed as an option, just below "Pin tab". Rather than add site-muting as an additional choice, "Mute tab" will be renamed, as described in Chrome's Git log:
Change Mute Tab to Mute Site on the Window menu on Mac
Currently, the Window menu shows "Mute Tab", which used to match the tab dropdown's "Mute tab". However, the tab dropdown text has been changed to "Mute site" for site muting, so we want to change the Window menu text to "Mute Site" to match.
Interestingly, site/tab muting didn't work for internal Chrome pages (those that begin
chrome://) — this has been rectified in version 64 also. At first, I couldn't recall any such tab being audibly offensive... or audible at all.
Then I saw this commit note, again, from Chrome's Git log:
We landed this with the intention of not allowing muting for any chrome:// URLs, but we've now changed our minds due to crbug.com/787743 (unable to mute chrome://dino tab).
Dino? Not sure what that is? Just type
chrome://dino (if you're using Chrome, of course) and it all make sense.