For classification of heart sounds, the wavelet domain has proven to be more promising. You may want to look at the "Classifying heart sounds workshop" in 2012, of which 2 of the 3 finalists used wavelet transformations (links here: http://www.peterjbentley.com/heartworkshop/#results). Especially, this paper covers some more detail: www.peterjbentley.com/heartworkshop/challengepaper3.pdf