1. A sound signal analysis apparatus comprising: sound signal input portion for inputting a sound signal indicative of a musical piece; 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 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.