Almost everyone can sing, but few people believe they can. Consequently, few people actually sing. It's more a matter of attitude than ability.
What is singing, anyway? Well, it's not speaking, that's for sure. Singing uses a different part of the brain than speech. People who stutter uncontrollably can sing just fine. People who cannot speak due to strokes can sometimes sing. People with Alzheimer's disease who can barely remember their own names can sometimes be capable of song.
Singing is different from speaking in a number of ways. Speaking is a series of consonants separated by vowels. Consonants are emphasized, and vowels are simply there to smooth the transition between consonants. Singing is the opposite. Singing is a series of vowel sounds separated by consonants. The vowels are elongated while the consonants are de-emphasized.
Imagine you have a dog named Barney. Say his name. "Barney." Now imagine Barney is outside and you are calling him. "Baaaaaarneeeeeeeey." The first is speech, the second is singing. That's all there is to it. Really.
Well, actually there is quite a lot more to it, but most of what people lack can be learned and developed over time. For most people the major hurdle is the mental one. They think they can't sing, and their lack of faith in themselves makes them shy. They develop weak, breathy, tremulous singing voices. Yet when they call the dog, they do it with power.
I see this in church all the time. People with powerful speaking voices can barely raise their voices in song. The problem isn't physical, it's mental. People haven't grown up singing and they are unsure of themselves. They have psyched themselves out.
This mental block is due, in part, to the growth of electronic media. In days gone by, people created their own entertainment. They got together in groups, gathered around whatever instrument was available, and sang songs. People sang their way through their chores. They sang their way through their joys and sorrows. They sang songs to teach their children. They sang songs to put their children to sleep. Their children sang songs at play. Singing was part of everyday life.
Then came records, radio, movies, & television. Suddenly the world's greatest entertainers were available almost on demand. Instead of making their own entertainment, people could simply purchase it. Why listen to the warblings of your own friends and family when they pale in comparison to the greats?
Good voices are everywhere, but few people believe they posses one. Great voices, by contrast, are quite rare. Great voices are freaks of nature, kind of like extreme intelligence or being seven feet tall. If great voices conferred some reproductive advantage, their would be more of them. But great voices are few, so few that they stand out. In the days before electronic media, people could go their entire lives without ever hearing one of these freakishly gifted folk. Now, the record shops are full of them. It makes it seem like we, the merely adequate, are somehow lacking, being born with a voice that is merely "good" isn't good enough. I'm not seven feet tall, and I'm fine with it. I can't sing like Sinatra, and I feel the ache of it. Odd.
While you and I could never posses a voice like Pavarotti, and while we may never make women swoon like the Beatles, we certainly could learn to use the voices we've been given. It's simply a matter of believing in ourselves and not feeling bad because we can't be great.