This NSApplescript toggles between the Internal Speakers and Apple TV. Could be saved as a service and triggered by keyboard shortcut or as an Alfred workflow

toggle-audio-output.scpt

(*
-- Change audio output between two outputs
-- Bind to a keyboard shortcut or an Alfred keyword
--
-- @author Einar Löve <einar@unfold.no>
-- @link http://github.com/einarlove
*)

set defaultOutput to "Internal Speakers"
set alternativeOutput to "Apple TV"

tell application "System Preferences"
	reveal anchor "output" of pane id "com.apple.preference.sound"
	
	tell application "System Events"
		tell process "System Preferences"
			
			set outputList to table 1 of scroll area 1 of tab group 1 of window "Sound"
			
			set currentOutput to value of text field of UI element 1 of outputList whose selected = true
			set outputName to first item of currentOutput
			
			if outputName is defaultOutput then
				set outputName to alternativeOutput
			else
				set outputName to defaultOutput
			end if
			
			select (row 1 of outputList whose value of text field 1 is outputName)
		end tell
	end tell
	
	quit
end tell

Thanks a bunch for this!