AppleScript for controlling Airfoil Volume/Mute from the command line with Karabiner

AirfoilVolumeControl.scpt

(* pass parameters to the script from the terminal to determine action *)

-- $ osascript AirfoilVolumeControl.scpt up
-- $ osascript AirfoilVolumeControl.scpt down
-- $ osascript AirfoilVolumeControl.scpt mute

(* muting requires enabling assistive access for the parent process since there's no AppleScript method for muting Airfoil without storing persistent variables *)

(* you can also remove the 'on run action' section of the script. This still allows you to invoke the adjustVolume function to directly manipulate the volume level *)

on run action
	set action to (action as string)
	if action is "mute" then
		tell application "System Events" to tell process "Airfoil"
			click menu item "Mute" of menu 1 of menu bar item "Speakers" of menu bar 1
		end tell
	else if action is "up" then
		adjustVolume(0.02)
	else if action is equal to "down" then
		adjustVolume(-0.02)
	end if
end run

on adjustVolume(change)
	tell application "Airfoil"
		set activeSpeakers to every speaker whose connected is true
		repeat with thisSpeaker in activeSpeakers
			set currentVolume to volume of thisSpeaker
			set (volume of thisSpeaker) to (currentVolume + change)
		end repeat
	end tell
end adjustVolume

private.xml

<?xml version="1.0"?>
<root>
  <vkopenurldef>
    <name>KeyCode::VK_OPEN_URL_Airfoil_Volume_Up</name>
    <url type="shell">
      <![CDATA[ osascript /AirfoilVolumeControl.scpt up ]]>
    </url>
  </vkopenurldef>
  <vkopenurldef>
    <name>KeyCode::VK_OPEN_URL_Airfoil_Volume_Down</name>
    <url type="shell">
      <![CDATA[ osascript /AirfoilVolumeControl.scpt down ]]>
    </url>
  </vkopenurldef>
  <vkopenurldef>
    <name>KeyCode::VK_OPEN_URL_Airfoil_Volume_Mute</name>
    <url type="shell">
      <![CDATA[ osascript /AirfoilVolumeControl.scpt mute ]]>
    </url>
  </vkopenurldef>

  <item>
    <name>Airfoil Volume Controls</name>
    <appendix>Left Control + Volume Up turns up active speakers</appendix>
    <appendix>Left Control + Volume Down turns down active speakers</appendix>
    <appendix>Left Control + Volume Mute mutes/unmutes active speakers</appendix>
    <identifier>private.airfoil.volume</identifier>
    <autogen>__KeyToKey__ ConsumerKeyCode::VOLUME_UP, ModifierFlag::CONTROL_L,
                          KeyCode::VK_OPEN_URL_Airfoil_Volume_Up</autogen>
    <autogen>__KeyToKey__ ConsumerKeyCode::VOLUME_DOWN, ModifierFlag::CONTROL_L,
                          KeyCode::VK_OPEN_URL_Airfoil_Volume_Down</autogen>
    <autogen>__KeyToKey__ ConsumerKeyCode::VOLUME_MUTE, ModifierFlag::CONTROL_L,
                          KeyCode::VK_OPEN_URL_Airfoil_Volume_Mute</autogen>
  </item>
</root>