Is Cmd+F10 that mute the Mac a built-in OS X keyboard shortcut? I just discovered that the combination of Cmd+F10 (without any Fn key pressed) muted my mac. I wanted to know if it was a built-in keyboard shortcut as it does not appear in the System preferences.
If you have 'Use all F1, F2 keys as standard functions' unchecked, that's the behaviour.
With or without the Cmd key, F10 mutes. Incidentally, so does Ctrl/F10 or Shift/F10.