If I understand correctly, you have an audio file with a singer over a backing track & you want to isolate either the singers voice or the backing track?
You can't really do this to a high quality....even with professional applications such as Cubase or Logic, it's extremely difficult.
All you can do if you've got a stereo audio mix is isolate certain frequencies and either boost or reduce them.
For example, you would need to determine the singers Vocal range
] which should be between 80Hz to 1100Hz
You could then set an extremely narrow filter over the frequency range that would cut out all the other audio.
While you get a result like this, it's not very good. Some of the original harmonics before the filter was applied always remain, and the filter itself will affect the audio you've isolated.
Basically, you want to get the eggs back after you've baked the cake!
Linky, found this
That's all they're doing there, setting a couple of band pass filters to narrow the frequency range