Google has finally added a mute button to the Chrome Global Media Controls.
The button is a stand-in for the mute button which used to be in the Picture-in-Picture button, but which Google removed.
The Global Media Control also supports a volume slider and device selector, but these are only available when casting media.
These improvements are available only in Chrome Canary, but will hopefully make their way to the mainstream client in a few weeks.
via Leo Varela