Shortcut to mute tabs in Safari
You'll have to create your own keyboard shortcut that toggles the menu bar commands under Window (otherwise you'll see that there are no keyboard shortcuts listed there).

Keyboard Settings - Shortcuts - App Shortcuts - "+" - Safari, Mute This Tab, (insert desired shortcut)

Repeat for "Mute Other Tab" and "Mute Other Tabs" (multiple "other" tabs open).



NOTE: Play/Pause button works for YouTube (pauses last video interacted played/paused), possibly other sites as well.



Right clicking the sound icon in the address bar will give you this functionality too, but requires the mouse.