Sorry about that
I figured I would divide this problem into three categories
1. speed of sound: The value of SoS depends upon the MEDIUM, and is affected by temp. humidity and winds. its transmission rate depends upon the elasticity of the medium [the Bulk and Young's modulus] (5X faster in steel than air). because sound travels as longitudinal waves (compressions and rarefactions) if the media is dense (humidity) or warmer (there more energetic) molecules to push around and SoS increases. As you get colder SoS decreases
2.mach number/TAS/EAS/IAS: as every one has said, increases with TAS which increases with altitude for a constant EAS, and increases as SoS decreases
3. gas laws: to avoid being timed out i'll post now
I feel I've not explained well
moles are a measure of the amount of a substance. Carbon 12 weigh 12 grams for every mol (n) and one mole of carbon has 6.023*10^23 C atoms. one mole of H2 gas a molecule weighs 2g but has same number of particles(Avogadro's No [NA]).
for an ideal gas (no molecular interaction)
The number of moles is dependent on 'state variables' P,V, and T. with any of these state variables one can find another unknown varialble, so the gas laws allow one to explain how SoS depends on any thing you want because all of the knowns lets say P,V and n can be used to rewrite the expression any how you'd like the above was a little algebra to relate every thing to density instead.
with out these laws thermodyamic would be speculation-at least till I build that entropy meter
lastly. I forgot this b4 Sigma = rho/rho(msl)