A stone is dropped into a well one hundred metres deep. The sound is heard 4.81 seconds later.

speed = distance/time but you have to deduct the time for the stone to fall.

en.wikipedia.org/wiki/Equations_for_a_falling_body
t = √(2d/g) =√(200/9.8) = 4.52 seconds

s = 100m/(4.81-4.52)s = 100m/0.29s = 344.8m/s

Speed varies according to temperature and humidity, so this answer is an average figure for that particular experiment. The speed of sound in dry air at 20 °C = 343 m/s.