1) Why are you comparing numeric values with a string comparison operator? Don't use if(%triggerVal $= 1), instead use if(%triggerVal == 1).
2) In order to make sure they're clicking the horn, you'll need to check if the %triggerNum argument (what you called it) is equal to 0, which is the fire weapon (left click by default) button, I believe.
3) The schedule method of objects doesn't require the second argument present in the schedule function. So, you just need %obj.schedule(500,playaudio,2,hornMiddle);, assuming those are the right argument values.