1. A method of diagnosing obstructive sleep apnea (OSA), the method comprising: acquiring a sleep sound signal comprising sounds made by a person during sleep; detecting a plurality of snore sounds in the sleep sound signal; determining a set of mel-frequency cepstral coefficients for each of the snore sounds; determining a variance for each of the sets of mel-frequency cepstral coefficients; summing the determined variances; determining a characterizing feature for the sleep sound signal responsive to the sum of the variances of the cepstral coefficients; and using the characterizing feature to diagnose OSA in the person, wherein the characterizing feature comprises a numerical value used as an indication of severity of OSA, where a low value means no OSA or a clinically insignificant case of OSA, a medium value means mild OSA, and a high value means severe OSA.