1. A sound signal processing apparatus for identifying a loop region where a similar musical character is repeated in a sound signal, said sound signal processing apparatus comprising: a character extraction section that divides the sound signal into a plurality of unit portions and extracts a character value of the sound signal for each of the unit portions; a degree of similarity calculation section that calculates degrees of similarity between the character values of individual ones of the unit portions; a first matrix generation section that generates a degree of similarity matrix by arranging the degrees of similarity between the character values of the individual unit portions, calculated by said degree of similarity calculation section, in a matrix configuration, said degree of similarity matrix having arranged in each column thereof the degrees of similarity acquired by comparing, for each of the unit portions, the sound signal and a delayed sound signal obtained by delaying the sound signal by a time difference equal to an integral multiple of a time length of the unit portion, said degree of similarity matrix having a plurality of the columns in association with different time differences equal to different integral multiples of the time length of the unit portion; a probability calculation section that, for each of the columns corresponding to the different time differences in the degree of similarity matrix, calculates a repetition probability indicative of a level of similarity on the basis of the degree of similarity; a peak identification section that identifies a plurality of peaks in a distribution of the repetition probabilities calculated by said probability calculation section; a second matrix generation section that generates a reference matrix having a plurality of columns corresponding to different time differences equal to different integral multiples of the time length of the unit portion and having predetermined reference values arranged in the columns associated with positions of the time differences where the plurality of peaks identified by said peak identification section are located; and a collation section that identifies the loop region in the sound signal by collating the reference matrix with the degree of similarity matrix.