About how fast is the speed of sound in air

The depends on the temperature, pressure, density, and humidity of
the air, so it's different in different places and at different times.

At sea level, with standard conditions of temperature and humidity
which I'm not going to take the time to look up right now, it's about 
340 meters per second.

That's like 760 miles per hour, or about  4.7 seconds per mile.