http://www.avatar.com.au/courses/PPofM/sound/sound4.html
It's a change of a change. If you calculate the change of Velocity of sound speed w.r.t. temperature(from 0C to 20C) the change of Velocity is approximately 11.5m/s. Divide this number over a change of 20 degrees, and you get approx. 0.575(m/s/degreeC), or approx 600 mm/s/degC.(forgot the /s in the last dimension statement)