OS X 10.11.1 El Capitan has disabled the keyboard sound keys
What happened here is that your FN key settings got reverted - there's an option that the F-keys either transmit their assigned key (so F10 means F10), or the additional function (so F10 means mute).

Normally, this is set to function mode, so F10 means mute. You can change this in your Keyboard settings in system preferences.



If the "Use as standard function keys" option is turned on, you will need to hit Fn to access the mute key. If it's turned off, you will not need to hit Fn, but if you want to send an actual F10, then you need to hold Fn.