Likelihood
One can match one song (A) against the timbre model of another
song (B), by computing the "probability of the data given the
model" (likelihood), i.e. computing the probability that the MFCCs
of song A be generated by the model of B, using the formula given
in (2). This is the most precise and logical way to compute a
distance, but it requires to have access to song A's MFCC, which
are relatively heavy to compute and to store.