First try doing a little geometry. Get out some graph paper and your calaculator
[with sine,tangent keys], and figure out the time difference in sound impingement
on 2 microphones placed about an owl's head size apart. And assuming sound
velocity is about 1100 ft/sec.
You will find the time difference is about 300 usec max [sound source to the side],
and much less for angles towards the front. If you do envelope detection differencing
as a first cut at a measurement, you'll probably find the Arduino A/D converter is a
little too slow for the job.