Clustering of Songs (The Joe Walsh Problem)
What you want to do seems to be known as Audio Feature Extraction or, more specifically, Music Information Retrieval, that is automated methods that distill characteristics out of (sets of) music files. You would have to extract features of samples of both equivalence classes and look at differences that can inform song choice.

Researchy tools are available for instance here and here. A finalist of Google Science Fair 2011 presented interesting stuff but I can not find his tools anywhere.