I'm gonna say no. Especially if they don't know they sing out of tune unless someone is telling them; becuase then it means to me they can't even distinguies what's in and out of key. And I don't believe that's something that can be learned. There are a few people I've known who occassionally sing out of key, but I think that's usually because they can't hear themselves well and once they find the solution to that, all is good. If given some quiet acoustic instruments however, if someone can't nail a tune vocally, I think they ought to hang it up and move onto something else. I invite your arguments as the above is my experience and not any kind of scientific fact. And if you're one of the people who used to not sing in key, and learned how, then I want video proof because I'm going to guess you now just think you sing in key.