As for the arrival time, I am referring to the difference in time for the sound to reach the two microphones.
You have an oversimplified view of a "sound". All sound is a continuously oscillating set of pressure variations, with discrete sounds having an envelope with a rise and a fall time, generally with lots of amplitude oscillations between start and finish. See this summary: Nine Components of Sound
If you define a certain positive going amplitude as the threshold for the "start" of a sound, then by definition the volume of the sound, which is related to the peak amplitude of the pressure wave, will determine whether that sound is detected.
The task you have chosen is actually quite difficult, so you might want to hold off a bit on that Instructable.