where did you get: Vsound in air = 331.4 + 0.6 Tc m/s?

The way i've learned the sound of minor pressure disturbances (so.. e.g. sound) is by this formula:

a = sqroot(yRT), where R = 287.05, T = temperature, and y (lamda) = 1.4

then you calculate the mach number by dividing the actual airspeed by this 'speed of sound':

M = V/a

So as you say, 'Vsound in air' is only a function of temperature, but i'm not sure whether 331.4 + 0.6Tc is correct.. it might be approximately correct but i'd advise to use the top formula.