A mute shortcut key is needed.
This is because pressing the speaker button only opens the volume bar and does not mute the sound in a single action. The volume bar must be manipulated each time.
At the very least, we think there should be a mute shortcut on the M key. Or it would be desirable to be able to mute a video by middle-clicking on it.
Also, if there is already a workaround, please let us know. My apologies.