Several paid cloud services were suggested by the other users for vocal isolation (and vocal/instrumental removal for music), but if you want a free and open-source program (that might even be used by the paid cloud services in question), I would strongly recommend Spleeter. It runs smoothly even on my old 2014 Mac mini. https://github.com/deezer/spleeter
Update: I don't recommend Spleeter anymore. I now recommend Ultimate Vocal Remover, which not only has a GUI, but has better sound and vocal separation quality. https://github.com/Anjok07/ultimatevocalremovergui