Interestingly, with a Nexus 5 and a certain pair of earbuds, this seems to happen automatically (minus the last step) - with most songs, I can hear the backing

Interestingly, with a Nexus 5 and a certain pair of earbuds, this seems to happen automatically (minus the last step) - with most songs, I can hear the backing tracks, but no vocals. The earbuds are passive components, so I assume there's something going on electrically that's combining the stereo channels somehow.