The speed of sound at sea level is 661.4748 knots.
It varies with temperature, which varies with altitude.
.
a=a0*(t/T0)^.5
a0 speed at sea level 1116.44 ft/sec
T0 absolute temp at sea level, deg K 288.16
The lapse rate with altitude is :
t=15-.0019812*pressure altitude.. deg C
or
t= 59-.00356616*pressure altitude.. deg F