I think you guys are talking about something different from tone deafness, are you talking about people who can't sing in tune?

I don't think it's about being tone "deaf" it's about being tone "dumb"-I apologize for the use of the word dumb. Just because you can't sing music, doesn't mean you can't play it. In my experience a great deal of musicians who play instruments can't sing very well. You can have both, but a lot of the times you usually have one of the other. So it isn't about not being able to hear the tune, or even recreate it on something else such as an instrument. It's about being able to "speak" aka sing it. People can whistle, people can tinker on the piano, everyone posses some ability to express music and rhythms in some way, even deaf people. (Beethoven wrote some of his greatest pieces while deaf)

Singing is solely based on biological factors, you can't teach someone to make a noise in their throat if they have no ability to do this. Or you can teach them the technical process of this, but it's not going to make them sing in tune. Singing is not "talking" it's a specific type of sound your larynx makes. When you sing, your larynx acts like a reed and creates a sound, our vocal chords allow us to manipulate that sound. (Of course all due to air, which creates the sound waves) This is what singing is, just speaking is not necessarily manipulation of your vocal chords. (Often times people just speak in the same tone-although good voice actors probably have the ability to manipulate while speaking) People who are tone deaf aka dumb, have no real ability to manipulate like a singer does.

I suppose you can look at it this way, everyone possess the reed, but not all of us our instruments. Not everyone is physically able to manipulate their vocal chords to match musical pitches. There are disorders in which people have medically lost their ability to be unable to possess pitch, such as Amusia. Of course actual deafness doesn't allow someone to hear pitch.

There are people who of course are just singing incorrectly, but usually you can hear some type of a resemblance of music. In reality there are actual people who simply have no biological ability to sing musically in pitch. My mother is one of them and it sounds like she can't physically sing the notes in tune, rather than she has no ability to distinguish pitch. We used to watch American Idol when it first came out and she could tell that the people who auditioned could not sing in tune.