I'm no audio expert, but I'm going to guess that's not possible unless the voice is literally a second track within the file. If it is possible, it's going to take some advanced tools that aren't free.
In any case, Audacity is the best free audio editor I know of. At best you might be able to filter out a range of frequencies, targeting the voice. You'll likely strip away some of the beat as well though - unless there's actually no crossover, which I find unlikely.
There's
this tutorial, but it hasn't worked all that well on the tracks I've tried. Or rather, I don't like how it pretty substantially changes the way the music sounds. You lose stereo separation, you lose bass, it sounds noisy and confusing, and you can still hear the vocals slightly. Maybe it works better on some tracks than others.