I had no idea about this. There's a whole little subculture of remixing old game music. For example there's this group which I guess does both real game music and remixes.
There's even various players for playing rips of the old game music like this one which plays NES music or this one which can play just about anything. I was listening to the music from Streets of Rage II on it, one of the soundtracks that made a game.