Melody/Vocal extraction
I remember reading a similar question (either here or on stack overflow) abouylt melody extraction. In any event someone did create such a project as a PhD thesis (luckily I bookmarked the site). He even offers his work (for research/personal/non-commercial use) for windows mac and linux http://www.justinsalamon.com/melody-extraction.html#demo if you could somehow integrate this to your application you can use the extracted melody to display the visuals, but still just play the original song from speakers/headphone