Sorry, Moira, I was never told to just mouth the words but I figured it out for myself. I can tell I'm not singing the same notes as everybody else (or as an instrument), but I can't tell whether I'm too high or too low.

To answer the original question, I think everybody can learn to improve their singing. I could probably even learn to sing along with others if I were willing to spend the time training myself or getting the training. Maybe I could even learn to hit the final tonics, which for some reason are the hardest part.

On technique and emotion: Bob Dylan is said to sing with a lot more feeling than Joan Baez, but Baez's best songs move me a lot more. Partly it's her beautiful voice, and partly it's her restraint. If the emotion is in the music, then you "just" need to sing the music--expressively, but not emoting--and I for one will get it. If the emotion's not in the music for you, then you should probably be considering performing other songs (and you probably are).