The Audio Volume System Tray applet can be middle-clicked to mute and unmute the system, but this is a hidden feature you would never discover if you didn't read Kai's blog. :) The feature is quite handy so let's expose it to more users by mentioning it in the tooltip subtext.
Please keep conceptual discussion ("do we really want to do this?") in plasma-workspace!2205 (merged) and only use this MR for design and implementation discussion. Thanks!