A few days ago I decided that Gollum's Song from The Two Towers (my favorite Lord of the Rings track) was physically impossible to sing as it appears in the movie. It's performed by Emiliana Torrini, who has an insane voice, the melody is nuts, and the range is way too large for me. (That, and it's an octave or two above where I can successfully sing.)
On the way home from work tonight, I have rescinded this judgment. I no longer that the song is impossible; just extremely difficult. After listening to it several times I can get the melody fairly well down (just at a much lower pitch), but I still can't imitate Emiliana's voice. She sounds a bit like Björk, but a little prettier. I'm not sure if there are many people in the world who could imitate her voice. And that's why I think that it's almost impossible for anyone else to perform that song.
A couple months ago I was asked by a choir director if I had ever been in a choir; I told him I had; I was in a professional children's choir for a year. He asked me why I wasn't in a choir any more; I told him that I could no longer sing. He replied, "you can't lose your ability to sing!" and the person standing next to him agreed. I replied simply with the word "puberty," to which he responded, "oh... guess that could do it." I used to sing Soprano I. Sometimes, when you need to differentiate between "high" and "really freaking high," you break Soprano into Soprano I and Soprano II. I was the "really freaking high." I used to have a good range, and I'd never miss a note (my voice quality wasn't great, though). Now I'm lucky if I can sing a dozen different notes, and I miss them without really understanding why. Oh well. Nobody else has to hear me sing now... except people in the bus stop when I pass by.