how to find variance of audio signal?

i am working on cry samples of infants to classify jaundice and Apnea ailment(based on cry analysis). Using STFT pitch is tracked and pitch contour is plotted to study pitch variability over the time. graph is attached below. variations incase of Apnea are more compared to jaundice case. so i want to find its variation and based on this i can classify using ANN. is it possible? pls help me to find variance. audio files are in .wav format.please help