How to remove vocals from a song that was not originally recorded in stereo?

[RelientKfanatic]

I downloaded a song from an old musical (like from the 40's!) because I wanted to sing the song for karaoke. I was going to rip the vocals from it until I found out that almost ALL vocal-removing software requires the song to be recorded in stereo (which this one is not!) Is there any possible way to remove the vocals? Thanks!