Within the artistic medium, I'm in a toss up between it being fair game and still being somewhat cheap to make a buck.
I do kinda agree though in which if you think about it in terms of how artist at conventions will sell fan art just fine, selling fan music is about at the same level. Point that I hold is that generally the fan art is for a single person while music is distributed in so many ways its hard to keep track.
*shrugs* I guess if there is a market for it, it'll happen either way, but I do agree with at the least getting some type of feedback from the original company to the games itself and probably even putting out a share of any profits to them for being kind enough to even let the person sell their music to begin with.
Not that those people need the money, but it shows face that you do thank them for being the original and giving you material to work with.