I've been working with karaoke for a while. There is no way to reliably remove vocal from the song which would result in acceptable quality music.
There are certain ways to do it, the most popular relying on a fact that the voice is usually center-panned to both channels while most of the instruments are not. You can use, for example, Audacity to do that. It works well on some (but not all) songs as long as it is stereo, but the result music quality would hardly be acceptable in most cases, for two reasons:
It will remove everything center-panned, such as kick drums (which are always center-panned), some other drums, and even some instruments which are center-panned (piano, for example, often is) - so the result music would sound kinda dull.
It will not remove the voice completely as every recording adds some echo which is not fully center-panned, and it will still be there.
And this is NOT how the karaoke songs are created. They're created either by a musician band "in style of", or on computer. Neither makes sense if you need a popular song, as Amazon or karaoke-versions most likely would have it, and for 99c a song it won't worth an effort.
equalizer
or perhaps thebandreject
audio filters if you want to tryffmpeg
. – Orgy