tell application "System Events"
	if (name of processes) contains "iTunes" then
		set iTunesRunning to true
	else
		set iTunesRunning to false
	end if
end tell

if iTunesRunning then
	tell application "iTunes"
		if player state is paused then
			play
		else if player state is playing then
			pause
		end if
	end tell
end if

Checks to see if iTunes is running and is playing / pausing a song and then does the opposite if it is.

iTunes Play Pause

AppleScript