[HPsquared]

Speed of sound also depends on the temperature (i.e. speed of the gas molecules) which is high and variable. These kinds of coupled interactions make combustion instability really hard to model in CFD hence the importance of full-scale testing.

[extraduder_ire]

If you solve the formulas for calculating it, the speed of sound only depends on temperature. Mostly because temperature is dependant on pressure itself.

[HPsquared]

Temperature and molecular properties.. which guess what are also highly variable in combustion! Very messy stuff. Especially when you get into the actual nitty gritty of all the intermediate chemical species involved.