Every sound a human being makes has some kind of resonance or it would be inaudible. Classical singing REQUIRES that a singer learn to stay in the pocket of 2800 to 3200 Hz in order to be heard unamplified over an orchestra. Very high sopranos may not need to do that, however, as high range of the average pitches sung can be enough. It is also necessary to be able to generate about 110 decibels in climatic phrases, which is very loud. That usually take years of training even in a naturally strong voice and body. There is no requirement, however, that the voice be beautiful while sustaining these resonance frequencies, and in point of fact, some voices can make these resonances sounding really dreadful. Maria Callas could always be heard but as she got older her technical problems got worse and worse and she sounded worse and worse but you could certainly always hear her. If you think she was unique, go over to the Met some time and see for yourself.
Resonance is NOT a cause, it is a result. So much singing training confuses cause with effect. It asks for singers to do things deliberately that are not meant to be deliberate. What we do deliberately is make sound, that is, when we choose. We can also choose to be silent. We also can breathe in a deliberate manner and do other things such as open the mouth, smile, and frown. Most people, with normal voices, can also get louder and softer (without understanding any special kind of breathing), and go up and down in pitch (if that was not possible no one would be able to ask a question or make a statement). None of that has anything to do with resonance or beauty of tone, or being musical, or being an artist, or having motivation.
Singing training is meant to increase and enhance function of the areas of the body that effect sound-making. The source of the sound is the vocal folds in the larynx. The folds control both pitch and register quality. The amount of air in the lungs and the pressure on that air as it crosses the vocal folds determines volume (decibels or sound pressure level), and the shape made in the mouth and throat coupled together as a tube (the vocal tract) determine what we hear in the vowel as it is sustained. There are all kinds of ways to produce resonance in the human voice and the specific resonances that group together at the frequencies mentioned above (2800 – 3200 Hz) are called “The Singer’s Formant”. A formant can be thought of as a resonanting frequency of the vocal tract. There are five that are significant. The first two determine the vowel, the other three determine the resonance factors. The front part of the mouth and tongue determine which vowel we hear and the back of the mouth and throat, down to the level of the vocal folds, determines what we would call the “timbre” or “color” of the tone, and much of its resonant quality. When resonance lines up with harmonics there is a “boost” in the sound, giving it a “greater energy”. If you want to learn more, go read one of the many excellent books on voice science. Scott McCoy’s book is very good.
None of this has to do with beauty of tone, that is a separate issue. Nor does it include learning how to do this is in a sensible, reasonable manner in any kind of codified approach. AND in CCM, we have microphones for carrying power, so resonance is a very arbitrary capacity. I wonder how much resonance Peggy Lee or Mel Torme had? Wonder how much Willy Nelson has or Tom Waits? Does anyone care? Michael Jackson’s voice was so light that you might not have been able to hear him 20 feet away when he was singing. No one knows. Were any of these artists striving to be resonant while they sang? You would have to had asked them, but my guess is that they were not. They had other goals in mind.
This is one of the many reasons why classical training is often NOT a good idea for those who want to sing in CCM styles. Learning to sing “Caro Mio Ben” does NOT help you sing “Good Morning Baltimore” from Hairspray. Learning to sing “An Die Musik” by Schubert does not help you sing “Being Alive” by Stephen Sondheim. Classical training that teaches you some kind of useful breath support skills, helps make your voice stronger, have more pitch range, keeps it comfortable and helps you sound NICER can be useful, but you have to get a teacher who knows what to teach and how to adapt it to you and your particular voice and career needs. Learning to be more resonant can be helpful if you have a very weak voice but, if you are a budding Peggy Lee, maximized resonance ain’t what you want unless you also want to end up sounding more like Sarah Vaughn. If it ain’t broke, don’t fix it.