Couldn't the neo-Nazis just write new songs with new melodies and words that the police don't know yet, or just make a non-nazi-sounding song their new secret anthem? And also, couldn't any human police officer know if he or she is in fact witnessing a neo-Nazi music event without an app? It seems to me that the risk that such technology could eventually be used to limit free speech is far greater than the potential usefulness of such an app.