1. A sound signal analysis apparatus comprising: a non-transitory memory device; a processor; a sound signal input portion for inputting a sound signal indicative of a musical piece; a feature value calculation portion for calculating a first feature value indicative of a feature relating to existence of a beat in one of sections of the musical piece and a second feature value indicative of a feature relating to tempo in one of the sections of the musical piece; and an estimation portion for concurrently estimating a beat position and a change in tempo in the musical piece by selecting, from among a plurality of probability models described as sequences of states classified according to a combination of a physical quantity relating to existence of a beat in one of the sections of the musical piece and a physical quantity relating to tempo in one of the sections of the musical piece, a probability model whose sequence of observation likelihoods each indicative of a probability of concurrent observation of the first feature value and the second feature value in corresponding one of the sections of the musical piece satisfies a certain criterion, wherein the sound signal input portion, the feature value calculation portion, and the estimation portion are implemented at least in part by the processor executing at least one program recorded on the non-transitory memory device.