Beg to differ with PT 1 of 3
Speed of sound is 660kts at sea-level ICAN (ie 15deg C). It is then proportional to the square root of the absolute temperature.
Eg SoS at an SAT of -40deg C = 660 x sqr rt(233)/sqr rt(288)
Not-so - your's makes the SoS at ICAN sea-level 1159.2 kts and you are going backwards at negative SATs!!
Mike Lithgow would have been a bit pushed...................