The simple and often taught answer is that the speed of sound is directly proportional to the square root of temperature.
a = sqrt(gamma*R*T) = sqrt(gamma*R)*sqrt(T)
Generally, the factor sqrt(gamma*R) can safely be assumed constant. It will not be a significant factor in day to day operations. That's close enough to the Absolute Truth for flying aircraft.
However, if the water did indeed have time to condense/evaporate, the sqrt(gamma*R) factor would change and the simple relationship with only temperature would no longer hold true. Hence the original posters question.
J T, 'adiabat' would be a very direct translation from my mother tongue, where the word is (rather questionably, I admit) used to designate the adiabatic lapse rate or, more specifically, the chart depicting the adiabatic lapse rate. Ooops! Sorry about the confusion. I did blame it on management...
And again, this is not my area of expertise. Any corrections of facts are welcome!