Step 5: Really precise stopwatch
The easiest way (as pointed out by one of my students) is to click and drag from the beginning of the sound to the beginning of the echo. If you look near the bottom of the window you'll be able to see how much time you've highlighted.
For mine I get a time of 0.009057 seconds. I figure four significant figures is pretty good. Now I simply measure the length of my tube, which in this case was 1.548 m. Since the sound has to go there and back again I double this distance (almost everyone forgets to do this the first time).
Speed can be calculated by dividing distance by time.
So v = d/t; v = 3.096/0.009057
v = 341.8 m/s or 764.6 mph
The accepted value at sea level is 340.29 m/s (the value in my classroom can be anywhere between 340 m/s and about 348 m/s depending on the weather).