Configure Karabiner Elements to control itunes volume / pause itunes. Install Karabiner-Elements from https://github.com/tekezo/Karabiner-Elements. Download this gist and put in `~/.config/karabiner/assets/complex_modifications`, then go to "Complex Modifications" tab, press "+ Add Rule" and then click Enable next to the rules

gistfile1.txt

{
   "title" : "iTunes direct",
   "rules" : [
      {
         "manipulators" : [
            {
               "from" : {
                  "modifiers" : {
                     "mandatory" : [
                        "fn",
                        "shift"
                     ]
                  },
                  "key_code" : "f11"
               },
               "type" : "basic",
               "to" : [
                  {
                     "shell_command" : "osascript -e 'tell application \"iTunes\" to set sound volume to sound volume - 10'"
                  }
               ]
            },
            {
               "to" : [
                  {
                     "shell_command" : "osascript -e 'tell application \"iTunes\" to set sound volume to sound volume + 10'"
                  }
               ],
               "from" : {
                  "key_code" : "f12",
                  "modifiers" : {
                     "mandatory" : [
                        "fn",
                        "shift"
                     ]
                  }
               },
               "type" : "basic"
            }
         ],
         "description" : "shift-fn-f11/f12 volume changes itunes volume"
      },
      {
         "manipulators" : [
            {
               "from" : {
                  "modifiers" : {
                     "mandatory" : [
                        "fn",
                        "shift"
                     ]
                  },
                  "key_code" : "f8"
               },
               "type" : "basic",
               "to" : [
                  {
                     "shell_command" : "osascript -e 'tell application \"iTunes\" to pause'"
                  }
               ]
            }
         ],
         "description" : "shift-fn-f8 always pauses itunes"
      }

   ]
}