Dear god. I really wouldn't want anybody in my car to be doing karaoke! Especially me!

I guess you could put in a software filter to attentuate the vocal frequencies. It would affect the tone of the instruments as well though. Another way would be to try to use the fact that a lot of vocals are mono and centered. No idea how this would work with MP3s though as the two channels will have differences due to the perceptual encoding.

- Trevor