Anyone knows how to remove (most of the) music from a song and keep only the vocals? Is removing vocals easy or this could be harder?

Completely removing the vocals from the song is not possible.Try it doing it with audio editing software.Suggest
FlexiMusic Wave Editor.In this software using either "Band Pass/ Stop Filter or Channel Mix” command you can eliminate the vocals to some extend.

I can suggest an alternative way.

If you want to remove the vocals to practice, it is sometimes easier to rewrite the score using a Music Notation SW such as Sibelius or Finale and the make a midi/mp3 out of it.

But:

It is normally a lot of work
You need the score to copy the notes (unless you can tell them by ear)
you need the SW and it could be expensive (see a post about this on the flute forum)